06 Thermal Processes

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Thermal Processes

ENVI 1400 : Lecture 6


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ENVI 1400 : Meteorology and Forecasting 2

Radiation ProcessesIncoming solar radiation

342 W m 2

Reflected by clouds,aerosol & atmosphere

77

168

30

Reflectedby surface

Absorbed by surface

Absorbed byatmosphere

67

thermals

24

24 Evapo-transpiration

78

78 390 324

324 350

40

40 30

Surface radiation Absorbed bysurface

reflected solarradiation107 W m 2

back radiation

emitted byatmosphere

165

Outgoinglongwaveradiation235 W m 2


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Adiabatic Processes An adiabatic process is one in

which no energy enters orleaves the system .

Many atmospheric processes

are adiabatic (or nearly so) particularly those involving thevertical movement of air.

Air is a poor thermal conductor,and mixing often slow enoughfor a body of air to retain itsidentity distinct from thesurrounding air during ascent.

Near-surface processes arefrequently non-adiabatic.

Adiabatic Processes: Ascent of convective plumes Large scale lifting/subsidence Condensation/evaporation

within an airmass

Non-Adiabatic Processes: Radiative heating/cooling Surface heating/cooling Loss of water through

precipitation Addition of water fromevaporation of precipitationfalling from above


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Lapse Rate

Lapse Rate is the termgiven to the verticalgradient of temperature.

The fall in temperaturewith altitude of dry air thatresults from the decreasein pressure is called theDry Adiabatic Lapse

Rate = -9.8 C/km.

1 k m

9.8 C

Temperature

A l t i t u

d e

Dry Adiabatic Lapse Rate


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Condensation releaseslatent heat, thussaturated air cools lesswith altitude than dry air.

There is no single valuefor the saturatedadiabatic lapse rate . Itincreases as temperature

decreases, from as lowas 4 C/km for very warm,tropical air, up to 9 C/kmat -40 C.

Temperature

A l t i t u

d e

Saturated AdiabaticLapse Rate

Dry AdiabaticLapse Rate


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Pressure & Temperature A column of air has pressure

levels P 1, P 2, etc. If the column is warmed, the air

will expand and its density at

any given level decrease. The vertical interval betweenpressure levels increases, sothat at any given altitude thepressure in the warmer columnis greater than in the cooler.

N.B. since the total mass of airin the column is constant, thepressure at the surface does notchange

P 0

P 1

P 2

P 3

P 4

P 5

z cool P 0

P 1

P 2

P 3

P 4

P 5

warm


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H

L

cool warm warm

cold-core High weakens with height,may form a low aloft

H

H

Warm-core High intensifies with height

cool coolwarm

L

L

Cold-core Low intensifies with height

cool warm warm

L

H

cool coolwarm

Warm-core Low weakens withheight, may form a high aloft



Mid-latitude low-pressure cellshave colder air to the rear.

As a result, the axis of the lowslopes towards the colder air

L

Sea-level isobars500 mb contours

Cold low

Warm high



High pressure cells slopetowards the warmest air aloft.

The centre of the cell at 3000mmay be displaced 10-15

towards the equator.

Sea-level isobars500 mb contours

Warm high

H

Cold low



The Thermal Low Thermal lows result from the

strong contrast in surfaceheating between land and sea

Land heats up (solar radiation)and cools down (infra-redradiation) much more rapidlythan ocean large diurnalcycle cross-coast temperaturegradient

N.B. A thermal low results fromfine, clear, warm weather, andthus differs from thedepressions associated withcloud and bad weather.



1. Start with a horizontallyuniform pressure distribution.Solar radiation starts to warm

land. Air near surface iswarmed by land, convectionmixes warm air upwards andwhole boundary layer warms.

2. Air over land warms andexpands. Cant expandsideways, so column expandupwards produces high

pressure aloft.N.B. Surface pressure remainsconstant at this stage.

warm coolcool

H



3. Horizontal pressure gradientaloft drives a flow from overland to over ocean.

warm coolcool

H

4. Mass of air in column over landis reduced surface pressurefalls to produce a surface low.High pressure aloft weakens,

but is maintained by continuedheating at surface.Surface pressure gradientdrives flow from sea to land:the sea breeze.warm coolcool

H

L


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H

L

5. When solar heating stops,pressure driven flows act toequalize pressure, restoring

conditions to the initial uniformpressure field.

If land cools sufficiently atnight, the reverse situation canbe established.

Over large land masses theremay be insufficient time overnight for the sea breeze toreach regions far from thecoast, and a weak surface lowis maintained over night. Thisthen deepens during thefollowing days, and a heat lowmay be maintained for days orweeks, until synopticconditions change.warmcool

H

L

warm


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Sea Breeze Formation of local thermal

low over land, results in theformation of a sea-breeze

In-flowing cool air from seaforms a sea-breeze front aminiature cold front

Air ahead of the front isforced upward, contributing tothe formation of cumulus.

1000 mb975 mb950 mb

25 C 15 C


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Pressure as an indicator oftemperature

Because the depth of alayer of air increases as itstemperature increases, wecan use the difference inaltitude between twoconstant pressure levels asan indicator of the meantemperature of the layer.

Charts are usually producedof the depth of the layerbetween 1000 and 500 mb.

The layer depth is usuallyquoted in deca-metres (10sof metres)

A useful rule of thumb isthat for 1000-500 mb layerdepths less than 528 dm(5280 m) any precipitationwill fall as snow rather than

rain.


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SLP (mb) & 1000-500 thickness : 48hr forecast valid 0000 040922


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SLP (mb) & 1000-500 thickness (dm) : 36hr forecast valid 0000 040930


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SLP (mb) & 1000-500 thickness (dm) : analysis valid 0000 040930


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12

C

2 C

850 mb Temperature (2 C contours), RH (%), wind (m s -1) : analysis valid 0000 040930


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Surface temperature (2 C contours) and SLP (mb)(5mb contours) : analysis valid 0600 040930


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The Thermal Wind It is commonly observed that

clouds at different altitudesmove in different directions winds are in differentdirections.

The gradient of wind velocity(speed & direction) is calledthe (vertical) wind shear .

In the free air, away fromsurface (where friction effectscomplicate matters), the windshear depends upon thetemperature structure of theair.

The thermal wind is atheoretical wind componentequal to the differencebetween the actual wind at twodifferent altitudes.

Any two levels can be used,but unless otherwise stated thealtitudes of the 1000mb and500mb levels are usually used.

Note that the 1000mb levelmight be below sea level, andis usually within the boundarylayer and thus influenced byfriction effects at the surface.


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60

0

120

180HIGH

LOW

5760

5820

5700

5640

VG500 VT

LOW

HIGH

VG1000

5700

56405580500-1000 mb thickness

Contours of1000 mb surface

Contours of

500 mb surface


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ENVI 1400 : Meteorolog and Forecasting 25

Note that cold air is to the leftof the thermal wind vector(looking along wind) in thenorthern hemisphere, to theright in the southernhemisphere.

The decrease in temperaturetowards the poles results in awesterly thermal wind in theupper atmosphere in bothhemispheres.

The largest meridionaltemperature gradient occurs inmid-latitudes across the polarfront.

The thermal wind makes up asignificant component of the

jet-stream, located over theupper part of the polar front.

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06 Thermal Processes