68
GEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin Krüger (MetOs, UiO) Email: [email protected]

Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

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Page 1: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

1

GEF1100ndashAutumn 2016 13092016

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

Research interests

bull Stratosphere-ocean interactions bull Dynamics of the stratosphere-troposphere bull Stratospheric ozone and ozone depleting substances bull Influence of volcanic eruptions on climate earth system

Who am I

Prof Dr Kirstin Kruumlger

kkruegergeouiono

climate modelling

observations

ship expeditions

Volcanic excursion

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

6

Addition not in book

Motivation 1 Motivation

Marshall and Plumb (2008)

7

- Derivation of the general equation of motion - Different horizontal wind forms - General circulation of atmosphere ocean

Equations of fluid motions

bull Hydrodynamics focuses on moving liquids and gases

bull Hydrodynamics and thermodynamics are fundamental for meteorology and oceanography

bull Equations of fluid motions for the atmosphere and ocean are constituted by macroscopic conservation laws for substances impulses and internal energy (hydrodynamics and thermodynamics)

The characteristics of very small volume elements are looked at (eg overall mass average speed)

Derivation of hydrodynamics is based on the conservation of momentum mass and energy

8 hydro (greek) water

2 Basics

State variables for atmosphere and ocean

u Wind u = (u v w) in ms T Temperature in degC or K p Pressure in Nm2 = 1 Pa (1 hPa = 102 Pa = 1 mbar) Specifically for the atmosphere q Specific humidity in kg water vapourkg moist air Specifically for the ocean S Salinity in kg saltkg seawater (often the salt content is expressed in practical salinity unit psu = kg salt1000 kg seawater) 9

2 Basics

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 2: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Research interests

bull Stratosphere-ocean interactions bull Dynamics of the stratosphere-troposphere bull Stratospheric ozone and ozone depleting substances bull Influence of volcanic eruptions on climate earth system

Who am I

Prof Dr Kirstin Kruumlger

kkruegergeouiono

climate modelling

observations

ship expeditions

Volcanic excursion

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

6

Addition not in book

Motivation 1 Motivation

Marshall and Plumb (2008)

7

- Derivation of the general equation of motion - Different horizontal wind forms - General circulation of atmosphere ocean

Equations of fluid motions

bull Hydrodynamics focuses on moving liquids and gases

bull Hydrodynamics and thermodynamics are fundamental for meteorology and oceanography

bull Equations of fluid motions for the atmosphere and ocean are constituted by macroscopic conservation laws for substances impulses and internal energy (hydrodynamics and thermodynamics)

The characteristics of very small volume elements are looked at (eg overall mass average speed)

Derivation of hydrodynamics is based on the conservation of momentum mass and energy

8 hydro (greek) water

2 Basics

State variables for atmosphere and ocean

u Wind u = (u v w) in ms T Temperature in degC or K p Pressure in Nm2 = 1 Pa (1 hPa = 102 Pa = 1 mbar) Specifically for the atmosphere q Specific humidity in kg water vapourkg moist air Specifically for the ocean S Salinity in kg saltkg seawater (often the salt content is expressed in practical salinity unit psu = kg salt1000 kg seawater) 9

2 Basics

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 3: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

6

Addition not in book

Motivation 1 Motivation

Marshall and Plumb (2008)

7

- Derivation of the general equation of motion - Different horizontal wind forms - General circulation of atmosphere ocean

Equations of fluid motions

bull Hydrodynamics focuses on moving liquids and gases

bull Hydrodynamics and thermodynamics are fundamental for meteorology and oceanography

bull Equations of fluid motions for the atmosphere and ocean are constituted by macroscopic conservation laws for substances impulses and internal energy (hydrodynamics and thermodynamics)

The characteristics of very small volume elements are looked at (eg overall mass average speed)

Derivation of hydrodynamics is based on the conservation of momentum mass and energy

8 hydro (greek) water

2 Basics

State variables for atmosphere and ocean

u Wind u = (u v w) in ms T Temperature in degC or K p Pressure in Nm2 = 1 Pa (1 hPa = 102 Pa = 1 mbar) Specifically for the atmosphere q Specific humidity in kg water vapourkg moist air Specifically for the ocean S Salinity in kg saltkg seawater (often the salt content is expressed in practical salinity unit psu = kg salt1000 kg seawater) 9

2 Basics

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 4: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Motivation 1 Motivation

Marshall and Plumb (2008)

7

- Derivation of the general equation of motion - Different horizontal wind forms - General circulation of atmosphere ocean

Equations of fluid motions

bull Hydrodynamics focuses on moving liquids and gases

bull Hydrodynamics and thermodynamics are fundamental for meteorology and oceanography

bull Equations of fluid motions for the atmosphere and ocean are constituted by macroscopic conservation laws for substances impulses and internal energy (hydrodynamics and thermodynamics)

The characteristics of very small volume elements are looked at (eg overall mass average speed)

Derivation of hydrodynamics is based on the conservation of momentum mass and energy

8 hydro (greek) water

2 Basics

State variables for atmosphere and ocean

u Wind u = (u v w) in ms T Temperature in degC or K p Pressure in Nm2 = 1 Pa (1 hPa = 102 Pa = 1 mbar) Specifically for the atmosphere q Specific humidity in kg water vapourkg moist air Specifically for the ocean S Salinity in kg saltkg seawater (often the salt content is expressed in practical salinity unit psu = kg salt1000 kg seawater) 9

2 Basics

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 5: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Equations of fluid motions

bull Hydrodynamics focuses on moving liquids and gases

bull Hydrodynamics and thermodynamics are fundamental for meteorology and oceanography

bull Equations of fluid motions for the atmosphere and ocean are constituted by macroscopic conservation laws for substances impulses and internal energy (hydrodynamics and thermodynamics)

The characteristics of very small volume elements are looked at (eg overall mass average speed)

Derivation of hydrodynamics is based on the conservation of momentum mass and energy

8 hydro (greek) water

2 Basics

State variables for atmosphere and ocean

u Wind u = (u v w) in ms T Temperature in degC or K p Pressure in Nm2 = 1 Pa (1 hPa = 102 Pa = 1 mbar) Specifically for the atmosphere q Specific humidity in kg water vapourkg moist air Specifically for the ocean S Salinity in kg saltkg seawater (often the salt content is expressed in practical salinity unit psu = kg salt1000 kg seawater) 9

2 Basics

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 6: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

State variables for atmosphere and ocean

u Wind u = (u v w) in ms T Temperature in degC or K p Pressure in Nm2 = 1 Pa (1 hPa = 102 Pa = 1 mbar) Specifically for the atmosphere q Specific humidity in kg water vapourkg moist air Specifically for the ocean S Salinity in kg saltkg seawater (often the salt content is expressed in practical salinity unit psu = kg salt1000 kg seawater) 9

2 Basics

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 7: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Differentiation following the motion - Euler ndash Lagrange derivatives

10

2 Basics

Example wind blows over hill cloud forms at the

ridge of the lee wave steady state assumption eq cloud does not change in time

Eulerian derivative (after Euler 1707-1783)

120597119862

120597119905 fixed point in space= 0

Lagrangian derivative (after Lagrange 1736-1813)

120597119862

120597119905 fixed particlene 0

C = C(xyzt) eg cloud amount 120597

120597119905 partial derivatives other variables

are kept fixed during the differentiation

Leonhard Euler (CH)

Joseph-Louis Lagrange (I)

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 8: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Differentiation following the motion - Euler and Lagrange perspectives

T(Xt) T(Xt+1)

χ=Xt+1 Lagrange

Parcel of air

trajectory

Euler

T(x1t2) u(x1t2) T(x1t1) u(x1t1)

Fixed box

in space

Euler perspective The control volume is anchored stationary within the coordinate system eg a cube through which the air or the seawater flows through

Lagrange perspective The control volume is an air or seawater parcel that always consists of the same particles and which moves along with the wind or the current

11

2 Basics

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 9: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Think about examples in your everyday life for Eulerrsquos and Lagrangersquos differentiation Discuss in groups of 2 and give one example

Euler and Lagrangian differentiation

12

2 Basics

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 10: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Differentiation following the motion mathematically

13

2 Basics

For arbitrary small variations

120575119862 =120597119862

120597119905120575119905 +

120597119862

120597119909120575119909 +

120597119862

120597119910120575119910 +

120597119862

120597119911120575119911

120575119862 119891119894119909119890119889 119901119886119903119905119894119888119897119890 =

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 120575119905

Dividing by δt and in limit of small variations

120597119862

120597119905 119891119894119909119890119889 119901119886119903119905119894119888119897119890=

120597119862

120597119905+ 119906

120597119862

120597119909+ 119907

120597119862

120597119910 + w

120597119862

120597119911 = 119863119862

119863119905

119863

119863119905

119863

119863119905=

120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911 equiv

120597

120597119905 + 119958 ∙ 120513

Advection term of a quantity 119958 ∙ 120629 lthellipgt ldquo

δ Greek small delta infinitesimal small size

Insert 120575119909 = u120575119905 120575119910 = v120575119905 120575119911 = w120575119905

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 11: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Differentiation following the motion mathematically

14

2 Basics

Lagrangian (or material substantial or total) derivative (after Lagrange 1736-1813)

119863

119863119905 fixed particle equiv 120597

120597119905+ 119906

120597

120597119909+ 119907

120597

120597119910 + w

120597

120597119911

equiv 120597

120597119905 + 119958 ∙ 120513 (6-1)

ldquoTime rate of change of some characteristic of a particular element of fluid which in general is changing its positionrdquo

Eulerian derivative (after Euler 1707-1783)

120597

120597119905 fixed point

ldquoTime rate of change of some characteristic at a fixed point in space but with constantly changing fluid element because the fluid is movingrdquo

119958 = (uvw) velocity vector

120513 equiv120597

120597119909

120597

120597119910120597

120597119911 gradient operator

called ldquonablardquo

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 12: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Forward trajectories started 850 hPa - top between 2604-29041986 - bottom between 3004-05051986

Example nuclear reactor accident Chernobyl on 26041986 0130 am Trajectories (=flight path) are used for the prediction of air movement

15

2 Basics

Kraus (2004)

Differentiation following the motion - Lagrangian perspective

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 13: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

16

Where is the missing Malaysian Airline MH370 (8 March 2014)

GEOMAR press release from 1 September 2015

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 14: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

22 Basics - Mathematical add ons

- Nomenclature

- Vector operations

- Nabla operator

17

22 Basics - Mathematical add ons September 13 2016

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 15: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Nomenclature summary

bull δ Greek small delta infinitesimal small size

eg δV= small air volume

bull partial derivative of a quantity with respect to

a coordinate eg x

bull total differential derivative of a quantity

with respect to all dependent coordinates (xzy)

bull Lagrangian (or total) derivative

18

x

dtd

22 Mathematical add ons

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 16: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Vector operations - Multiplication with a scalar a -

z

y

x

z

y

x

ii

av

av

av

v

v

v

aavavavva

bull With a scalar multiplication the vector stays a

vector

bull Each element of a vector is multiplied individually with a

scalar a

bull The vector extends (or shortens) itself by the factor a

bull Convention With the multiplication scalar ndash vector we

donrsquot use a point (like with scalar ndash scalar)

22 Mathematical add ons

19

wind

vector

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 17: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Vector operations - Scalar product -

ii

i

iiiizzyyxx

z

y

x

z

y

x

vfvfvffvfvfv

f

f

f

v

v

v

vffv

bull The scalar product of two vectors is a scalar

bull It is multiplied component-wise then added

bull Convention The scalar product is marked by a multiplication sign (middot)

bull It is at a maximum with parallel vectors and disappears when the

vectors are perpendicular to each other

f

v

cosfvfv

22 Mathematical add ons

20

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 18: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Vector operations ndash vector product

bcacbabac

baba

baba

baba

babakbabajbabai

bbb

aaa

kji

abbac

xyyx

zxxz

yzzy

xyyxzxxzyzzy

zyx

zyx

sin

)()()(

a

b

bull The vector product of two vectors is again a vector

bull Convention The vector product or cross product is marked by an ldquoxrdquo

bull If is turned to on the fastest route possible the vector (from the

vector product) points in the direction in which a right-hand screw

would move (right hand rule)

bull With parallel vectors it disappears and it reaches its maximum when the

two vectors are at right angles to each other

21

22 Mathematical add ons

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 19: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Graphically

- Pooling of the spatial gradients towards the spatial coordinate axes rarr vector

- Gradient points towards the largest increase of the quantity

- Amount is the magnitude of the derivative pointing towards the largest increase

Nabla operator ndash spatial gradient

with

Operator -Nabla

z

y

x

i

22

bull Nabla is a (vector) operator that works to

the right

bull It only results in a value if it stands left of

an arithmetic expression

bull If it stands to the right of an arithmetic

expression it keeps its operator function

(and ldquowaitsrdquo for application)

22 Mathematical add ons

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 20: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

TTgradT

z

y

x

zT

yT

xT

z

w

y

v

x

u

w

v

u

udivu

z

y

x

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Vector product ldquoxrdquo

vector

Scalar product ldquordquo

scalar

Product with a

Scalar vector

Nabla operator 22 Mathematical add ons

23

Unit vectors

i x direction

j y direction

k z direction

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 21: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Nabla operator ndash algorithms

z

y

x

z

y

x

zT

yT

xT

T

T

T

TTT

zw

yv

xu

w

v

u

u

z

w

y

v

x

u

w

v

u

udivu

z

y

x

z

y

x

22 Mathematical add ons

Note Nabla is a (vector) operator ie the sequence must not be changed here

24

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 22: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Divergence Convergence

0 Hu 0 Hu

Characteristics of horizontal wind

25

22 Mathematical add ons

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 23: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Rotation and vorticity (comes from the word ldquovortexrdquo)

26

Rotation of horizontal flow

Horizontal

wind Rotation

L

Characteristics of horizontal wind

22 Mathematical add ons

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 24: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Rotation of a vector field - Vector product of the Nabla operator with a vector -

y

u

x

vx

w

z

u

z

v

y

w

wvu

kji

w

v

u

urotuzyx

z

y

x

Definition vorticity

ζ Greek bdquoZetaldquo

The vorticity is a scalar

The rotation of a flow field is constituted by

The vorticity ζ is defined as a vertical component of the velocity fieldsrsquos rotation

u u rot

y

u

x

v

-uk

22 Basics - Vorticity

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 25: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Relative vorticity ζ

Rotation relative to the earth

Absolute vorticity η

Sum of relative vorticity and planetary vorticity

Rotation in an absolute coordinate system oriented towards a fixed star

Planetary vorticity f

Contribution of earth rotation f = 2 Ω sinφ f Coriolis parameter φ latitude Ω angular velocity Ω = 2π24h is the angular velocity of the earth rotation

yx

uvuk

f

Application examples eg vorticity advection in synoptic strength of a vortex (eg stratospheric polar vortex) stratosphere vorticity correlates with ozone

ζ Zeta η Eta f Coriolis parameter

Vorticity

28

22 Basics - Vorticity

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 26: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Curvature vorticity (VKs)

Shear vorticity

H

L

Tropopause

STJ

Examples

STJ Sub-

tropical Jet

Vorticity (NH)

Note In general the horizontal wind field contains both the curvature as well as the shear forces (varying strengths of winds on the verticals)

x

y

z

ζ lt 0

ζ gt 0

Surface

22 Basics - Vorticity

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 27: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

NH stratospheric polar vortex

Potential vorticity unit (PVU) at ~20 km altitude

Stratospheric

polar vortex

Edge of vortex

Filament

- High vorticity (PV) in the centre of the polar vortex (low pressure centre) - Greatest PV gradient at the edge of the polar vortex (rarr wind maximum) 30

L

x Oslo

22 Basics - Vorticity

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 28: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

31

Addition not in book

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 29: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

I Laws of motion for a fluid parcel in x-y-z- directions (applying Newtonrsquos 1 and 2 law)

II Conservation of mass

III Law of thermodynamics (including motion)

rarr5 equations for the evolution of the fluid (5 unknowns

(u p T)) 32

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 30: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Momentum conservation The conservation of momentum is represented by

Newtonrsquos first law (ldquoLex primardquo)

(momentum = mass middot velocity)

momentum = const at absence of forces

Momentum sentence (ldquoLex secundardquo)

temporal change in momentum = Force

(mass middot acceleration = Force)

33

Fu

dt

dM t time M mass u velocity vector F Force vector

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 31: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

fluid parcel - cube

34

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 32: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Forces on a fluid parcel

35

120588 120575119909 120575119910 120575119911 119863119958

119863119905 = F (Eq 6-2)

120588 density (120575119909 120575119910 120575119911) fluid parcel with infinitesimal dimensions 120575119872mass of the parcel 120575119872 = 120588 120575119909 120575119910 120575119911 119958 parcel velocity F net force

119863119958

119863119905 =

120597119958

120597119905 + u

120597119958

120597119909 +v

120597119958

120597119910+w

120597119958

120597119911

= 120597119958

120597119905 + (119958 ∙ 120571) 119958

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 33: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Gravity

36

The gravitational force g 120575119872 is directed downward

Fgravity= minusg 120588119963 120575119909 120575119910 120575119911 (Eq 6-3)

Fgravity Gravitational force M mass g gravity acceleration g asymp const 120588 density 119963 unit vector in upward direction

Marshall and Plumb (2008)

3 Equations of motion - nonrotating fluid

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 34: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

37

x-component of the pressure force is

F(A) = p ∙ 119860 = 119901 (x minus 120575119909

2 y z) 120575119910 120575119911

F(B) = p ∙ 119861 = minus119901 (x + 120575119909

2 y z) 120575119910 120575119911

Fx = F(A) + F(B)

Apply Taylor expansion (A21) at midpoint of parcel and neglect small terms (see book)

Fx =minus120597119901

120597119909120575119909 120575119910 120575119911

Apply for all sides (y and z)

119901 pressure

pressure force

(inward)

pressure

force

(inward)

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 35: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

3 Equations of motion - nonrotating fluid

Marshall and Plumb (2008)

Pressure gradient force

38

Fpressure= 119865119909 119865119910 119865119911

=minus120597119901

120597119909120597119901

120597119910120597119901

120597119911120575119909 120575119910 120575119911

=minus 120571119901 120575119909 120575119910 120575119911 (Eq 6-4)

119901 pressure

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 36: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Friction force

39

3 Equations of motion - nonrotating fluid

- Friction force operates on the earthrsquos surface

- the greater the surface roughness the higher the friction force

- the greater the wind velocity the higher the friction force

- friction force takes effect in vertical distances of ~ 100 m up to 1000 m (rarr atmospheric boundary layer Chapter 7)

Ffriction = 120588 ℱ 120575119909 120575119910 120575119911 (Eq 6-5)

ℱ frictional force per unit mass (see book chapters 742 and 101) 120588 density

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 37: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

GEF 1100 ndash Klimasystemet

Chapter 6 The equations of fluid motion

40

Prof Dr Kirstin Kruumlger (MetOs UiO) Email kkruegergeouiono

GEF1100ndashAutumn 2016 15092016

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 38: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

41

Addition not in book

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 39: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Equations of motion

bull All three forces together in Eq 6-2 gives

120588 120575119909 120575119910 120575119911 119863119854

119863119905 = F119901119903119890119904119904119906119903119890 + F119892119903119886119907119894119905119910 + F119891119903119894119888119905119894119900119899

bull Re-arranging leads to equation of motion for fluid parcels

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Equation 6-6)

42

3 Equations of motion - nonrotating fluid

120588 density u velocity vector F Force vector ℱ frictional force per unit mass p pressure g gravity acceleration 119963 unit vector in z-direction

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 40: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Equations of motion ndash component form

(Eq 6-6) in Cartesian coordinates

bull120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

bull120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

bull120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

43

3 Equations of motion - nonrotating fluid

119906 zonal velocity 119907meridional velocity w vertical velocity 120588 density ℱ frictional force per unit mass p pressure g gravity acceleration

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 41: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Hydrostatic balance

If friction ℱ119911 and vertical acceleration 119863119908

119863119905 are negligible we

derive from the vertical eq of motion (6-7c) the hydrostatic balance (Ch3 Eq3-3)

120597119901

120597119911 = - 120588g (Eq 6-8)

Note This approximation holds for large-scale atmospheric and oceanic circulation with weak vertical motions

44

3 Equations of motion - nonrotating fluid

Balance between vertical pressure gradient and gravitational force ldquoPressure decreases with height in proportion to the weight of the overlying atmosphererdquo

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863119908

119863119905=

120597119908

120597119905+ 119906

120597119908

120597119909+ 119907

120597119908

120597119910 + w

120597119908

120597119911

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 42: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Conserved quantity A quantity of which the derivative equals zero

Conservation of mass for liquids is also called

continuity equation

Conservation of mass ndash continuity equation

45

4 Conservation of mass

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 43: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Conservation of mass

46

4 Conservation of mass

Marshall and Plumb (2008)

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 44: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

4 Conservation of mass

47

Conservation of mass -1

Mass continuity requires

120597

120597119905 (120588 120575119909 120575119910 120575119911) =

120597120588

120597119905 120575119909 120575119910 120575119911

= (net mass flux into the volume)

bull mass flux in x-direction per unit time into the volume

120588119906 119909 minus1

2 120575119909 119910 119911 120575119910 120575119911

bull mass flux in x-direction per unit time out of the volume

120588119906 119909 +1

2 120575119909 119910 119911 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 45: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

4 Conservation of mass

48

Conservation of mass -2

bull net mass flux in x-direction into the volume is then (employing Taylor expansion A22)

minus120597

120597119909120588119906 120575119909 120575119910 120575119911

bull net mass flux in y- and z- direction accordingly (see book)

bull Net mass flux into the volume

minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911 bull substituting into mass continuity

120597

120597119905 (120588 120575119909 120575119910 120575119911) = minus 120571 ∙ 120588119958 120575119909 120575119910 120575119911

Marshall and Plumb (2008)

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 46: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

4 Conservation of mass

49

Conservation of mass -3

bull leads to equation of continuity

120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (Eq 6-9)

which has the form of conservation law

120597 119862119900119899119888119890119899119905119903119886119905119894119900119899

120597119905+ 120571 ∙ 119891119897119906119909 = 0

bull Using DDt (Eq 6-1) and 120571 ∙ 120588119958 =120588120571 ∙ 119958 + 119958 ∙ 120571120588 (see A22)

we can rewrite

119863120588

119863119905+ 120588120571 ∙ 119958 = 0 (Eq 6-10)

Marshall and Plumb (2008)

119863

119863119905 119871119886119892119903119886119899119892119894119886119899 119889119890119903119894119907119886119905119894119907119890

119863

119863119905=

120597

120597119905 + 119958 ∙ 120513

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 47: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Approximations for continuity equation

incompressible - compressible flows

bull Incompressible flow (eg ocean)

120571 ∙ 119958 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119911= 0 (Eq6-11)

bull Compressible flow (eg air ρ varies) expressed in pressure coordinate p applying hydrostatic assumption

120571119901 ∙ 119958119901 =120597119906

120597119909+

120597119907

120597119910+

120597119908

120597119901= 0 (Eq6-12)

50

4 Conservation of mass

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 48: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Thermodynamic equation

bull Temperature evolution can be derived from first law of thermodynamics (Ch4 4-2)

119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (Eq 6-13)

bull If the heating rate is zero (119863119876

119863119905 = 0 Ch 431) then

119863119879

119863119905 =

1

120588119888119901

119863119901

119863119905

51

5 Thermodynamic equation

The temperature of a parcel will decrease increase in ascent descent with decreasingincreasing pressure

rarr Introduction of potential temperature 120579 (Eq 4-17)

120579 = 119879 1199010

119901

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

120588 density p pressure p0 = 1000 hPa κ=Rcp R gas constant for dry air

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 49: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Thermodynamic equation - potential temperature

bull Eq 6-13 for potential temperature 120579 (see book for details)

119863120579

119863119905=

119901

1199010

minusκ 119863119876

119863119905

119888119901

(Eq 6-14)

Note For adiabatic motions 120575119876 = 0 120579 is conserved

52

5 Thermodynamic equation

119876 heat 119863119876

119863119905 diabatic heating rate per unit mass

119888119901 specific heat at constant pressure T temperature

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 50: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

53

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 51: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Ia) 120597119906

120597119905 + 119906

120597119906

120597119909 + 119907

120597119906

120597119910 + w

120597119906

120597119911 +

1

120588

120597119901

120597119909 = ℱx (6-7a)

Ib) 120597119907

120597119905 + 119906

120597119907

120597119909 + 119907

120597119907

120597119910 + w

120597119907

120597119911 +

1

120588

120597119901

120597119910 = ℱ119910

(6minus7b)

Ic) 120597119908

120597119905 + 119906

120597119908

120597119909 + 119907

120597119908

120597119910 + w

120597119908

120597119911 +

1

120588

120597119901

120597119911 + g = ℱz (6-7c)

II) 120597120588

120597119905+ 120571 ∙ 120588119958 = 0 (6-9 or 6-116-12)

III) 119863119876

119863119905= 119888119901

119863119879

119863119905 minus

1

120588 119863119901

119863119905 (6-13 or 6-14)

54

Summary Summary 3-5)

rarr5 equations for the evolution of the fluid (5 unknowns)

Restrictions - application to average motion is often incorrect ie turbulence - fixed coordinate system

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 52: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

bull Eulerian ndash Lagrangian derivations perspectives

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

Take home message I

55

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 53: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

1 Motivation

2 Basics 21 Lagrange - Euler 22 Mathematical add ons Vorticity

3 Equation of motion for a nonrotating fluid 31 Forces 32 Equations of motion 33 Hydrostatic balance

4 Conversation of mass

5 Thermodynamic equation

6 Equation of motion for a rotating fluid 61 Forces 62 Equations of motion

7 Take home messages

Lect

ure

Ou

tlin

e ndash

Ch

6 Ch 6 - The equations of fluid motions

56

Addition not in book

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 54: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Forces on rotating sphere

Fictitious forces

- occur in revolvingaccelerating system

(eg car in curve)

- occur on the earthrsquos surface

(Fixed system on earthrsquos surface forms an accelerated system because a circular motion is performed once a day Movement of earth around the sun compared to earthrsquos rotation is insignificant)

rarr Coriolis force

rarr Centrifugal force

57

5 Equations of motion - rotating fluid

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 55: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Coordinate systems in meteorology

bull Cartesian coordinates (xyz)

bull Spherical polar coordinates (λφz)

bull Height coordinates

geometrical altitude (z)

pressure (p) and

potential temperature (θ)

3 Equations of motion - nonrotating fluid

a radius Ω Greek omega rotation vector spherical polar coordinates φ 119897119886119905119894119905119906119889119890 λ 119897119900119899119892119894119905119906119889119890 119911 119903119886119889119894119886119897 119889119894119904119905119886119899119888119890

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 56: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

59

Inertial - rotating frames

119955 position vector Ω Greek omega rotation vector

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 57: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

60

5 Equations of motion - rotating fluid

Transformation into rotating coordinates

Consider figure 69

bull 119958119894119899 = 119958119903119900119905 + Ω times 119955 (Eq 6-24) We set A =r The transformation rule for DDt acting on a vector is given by

bull119863119912

119863119905 119894119899=

119863119912

119863119905 119903119900119905+ Ω times A (Eq 6-26)

Then we set A rarr 119958119894119899 in Eq 6-26 using 6-24 we derive

119863119958119894119899

119863119905 119894119899=

119863

119863119905 119903119900119905+ Ω times 119958119903119900119905 + Ω times 119955

=119863119958

119903119900119905

119863119905 119903119900119905+ 2Ω times 119958119903119900119905 + Ω times Ω times 119955 (Eq 6-27)

119863119955

119863119905 119903119900119905= 119958119903119900119905

119955 position vector A any vector Ω rotation vector

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 58: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

61

5 Equations of motion - rotating fluid

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 59: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

62

Coriolis acceleration

119863119854

119863119905 = minus 2120512 times 119958 Eq 6-31

NH (viewed from above the Northpole) Ω gt 0 rotation anticlockwise SH (viewed from above the Southpole) Ω lt 0 rotation clockwise Absence of other forces

120512 =0 Ωy

Ωz

=0

Ω 119888119900119904φΩ sinφ

Angular velocity

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 60: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

5 Equations of motion - rotating fluid

Marshall and Plumb (2008)

63

Assuming that 1) Ωz is negligible as Ωu ltlt g 2) wltltuh leads to

minus2120512 times 119958 ≃(minus2Ω sinφ119907 2Ω sinφ 1199060) (Eq 6-41)

= 119891119963 times 119958 Coriolis parameter 119943

119891 = 2Ω sin120593 (Eq 6-42)

Note that Ω sinφ is the vertical component of the Earths rotation rate only component that matters

Coriolis force on the sphere ndash Coriolis parameter

Ω= 727 x 10-5 s-1 a= 637 x 106 m

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 61: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Coriolis force In direction of movement

in

NH

deflection of air

particles to the right

SH

deflection of air

particles to the left

takes place

Directional movement

64

Schematic picture for Coriolis force

5 Equations of motion - rotating fluid

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 62: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

0

0

cyclonic motion in

anti-clockwise direction low pressure

anticyclonic motion in

clockwise direction gt high pressure

Rotation Cyclonic ndash Anticyclonic

65

We call a rotating movement ldquocyclonicrdquo if it follows the same rotational

direction as the earth

As f gt 0 in the NH the rotary movement is

ζ vorticity

f Coriolis parameter

What is the rotary motion like in the SH

In the SH because of f lt 0 it is exactly the opposite

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 63: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Centripetal force FCp Centrifugal force FC

Angular velocity Ω

Body (mass M) Centre

FCp = - FC

Centrifugal force ndash Centripetal force

r

66

FC = minusM Ω x (Ω x r)

bull Directed radially outward

bull If no other forces are present the particle would accelerate outwards bull Fictitious force balanced by Centripetal force

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 64: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Rotating equations of motion

Substituting 119863119958119894119899

119863119905 119894119899 from Eq 6-27 into Eq 6-6 (inertial frame

equation of motion ) we derive in rotating frame dropping subscript ldquorotldquo 119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ (Eq 6-6)

119863119854

119863119905 +

1

120588120571119901 + g119963 = minus 2Ω times 119958 minus Ω times Ω times 119955 + ℱ (Eq 6-28)

67

5 Equations of motion - rotating fluid

Coriolis acceleration

Centrifugal acceleration

Friction acceleration

Pressure gradient accel

Gravi- tational accel

119958 velocity vector t time p pressure g gravity acceleration119963 unit vector in zminusdirection Ω rotation vector ℱ frictional force per unit mass

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 65: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Centrifugal acceleration ndash modified gravitational potential

Combine the gradient of Centrifugal potential minusΩ times Ω times 119955 = 120571Ω2r2

2 and

Gravitational potential g119963 = 120571(gz) in Eq 6-28

119863119854

119863119905 +

1

120588120571119901 + 120571120567 = minus 2Ω times 119958 + ℱ Eq 6minus29

where 120567 = g119911 minusΩ2r2

2 Eq 6minus30

120567(Greek ldquoPhildquo) is a modified (by centrifugal accelerations) gravitational potential ldquomeasuredrdquo in the rotating frame

68

Coriolis acceleration

Friction acceleration

Pressure gradient accel

Gravitational +Centrifugal accelerations

5 Equations of motion - rotating fluid

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 66: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Marshall and Plumb (2008) 69

5 Equations of motion - rotating fluid

hellipon the sphere ldquoShallow atmosphererdquo approx (a+z ≃ a)

r=(a+z)cos120593 ≃ a cos120593 hence Eq 6-30 becomes

120567 = g119911 minusΩ21198862cos2120593

2

Definition of geopotential surfaces

119911lowast = 119911 +Ω21198862cos2120593

2g (Eq 6-40)

Ω= 727 x 10-5 s-1 a = 637 x 106 m

rarrAt the equator 119911lowast 119894119904 Ω21198862

2g asymp 11 km

higher than at the Pole (119911lowast=z)

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 67: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

Equations of motion

119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ (Eq 6minus43)

bull Fluid in a thin spherical shell on a rotating sphere applying hydrostatic balance for vertical component and neglecting ℱ119911

compared with gravity

119863u

119863119905 +

1

120588

120597119901

120597119909minus 119891119907 = ℱ119909 (Eq 6minus44)

119863119907

119863119905 +

1

120588

120597119901

120597119910+ 119891119906 = ℱ119910

1

120588

120597119901

120597119911+ g = 0

70

5 Equations of motion - rotating fluid

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71

Page 68: Chapter 6: The equations of fluid motion - Forsiden - · PDF fileGEF 1100 – Klimasystemet Chapter 6: The equations of fluid motion 1 GEF1100–Autumn 2016 13.09.2016 Prof. Dr. Kirstin

bull Eulerian ndash Lagrangian derivations perspectives

bull 5 equations for the evolution of the fluid with 5 unknowns (uvwpT) equations of motion (3) conservation of mass (1) thermodynamic equation (1)

bull Equations of motion on a non-rotating fluid Pressure gradient force gravitational force and friction force act

119863119854

119863119905 +

1

120588120571119901 + g119963 = ℱ

bull Equations of motion on a rotating fluid Pressure gradient force

modified gravitational potential (gravitational and centrifugal force)

Coriolis force and friction force act 119863119854

119863119905 +

1

120588120571119901 + 120571120567 +f 119963 times 119958 = ℱ

Take home message I+II

71