Extratropical Cyclones

– Genesis, Development, and Decay

Xiangdong Zhang

International Arctic Research Center

Basic Facts

Extratropical cyclones is a major weather maker for mid and high latitudes.

Size: roughly 1000-2500 km in diameter;

Intense: central pressure ranging from 970-1000 hPa;

Lifetime: 3-6 days to develop, and 3-6 to dissipate;

Movement: generally eastward at about 50 km/hr;

Peak season: winter;

Formation: along baroclinic zone or from transition of tropical cyclones.

Goal: Understand cyclone from simple model to complex dynamics

Outline

• Classic surface-based polar-front model – Bergen Model

• Surface – upper troposphere coupling – understanding from kinematics

• Interactions between dynamics and thermodynamics – a more complex vorticity dynamics

Bergen Cyclone Model (BCM)

Mechanism of cyclone development: Baroclinic instability

Center ofGravity

hh ≈ 0

Baroclinic Instability: Available potential energy (APE) kinetic energy(air movement -> wind)

Warm

cold

Z

Unstable Stable

Unstable Stable

Are we satisfied with BCM so far?

• How do upper level waves disturb the surface cyclone formation?

Questions we could not answer:

• How can surface cyclone be maintained when air mass fills in?

How does ageostrophic wind redistribute air mass and links upper level waves to surface cyclone development?

planetary waves at 500 hpa a weather chart at 500 hpa

Surface – upper troposphere coupling

• Ageostrophic wind: difference between the actual wind and the wind when it is in perfect geostrophic balance:

• Geostrophic wind: the wind when it is in perfect geostrophic balance:

Force BalanceFree Atmosphere

component

componentAgeostrophic wind:

<0: cyclonic curving

>0: anticyclonic curving

Ageostrophic wind when the air curves cyclonically:

• The centripetal acceleration breaks the geostrophic balance;

• The ageostrophic wind points the opposite direction of the geostrophic wind.

Sub-geostrophic wind: slower than the geostrophic wind.

High Pressure

Low Pressure

Pressure Gradient Force

Coriolis Force

Centripetal Acceleration

Ageostrophic wind when the air curves anticyclonically:

• The centripetal acceleration breaks the geostrophic balance;

• The ageostrophic wind points the same direction of the geostrophic wind.

Super-geostrophic wind: faster than the geostrophic wind.

Low Pressure

High Pressure

Coriolis Force

Pressure Gradient Force

Centripetal Acceleration

Ageostrophic wind when the air speeds up:

• The pressure gradient increases and air blows toward lower pressure side;

• The ageostrophic wind points the left of the geostrophic wind.

Ageostrophic wind when the air slows down:• Opposite.

High Pressure

Low Pressure

Pressure Gradient Force

Coriolis Force

Summary I: Curvature effects (uniform pressure gradients along the flow)

PGF > CFP(PGF increases)

CF > PGF(PGF decrease)

Low Pressure

High Pressure

Coriolis Force

Pressure Gradient Force

old new

Convergence

Convergence

Divergence

Divergence

Summary II: Effects from varying pressure gradients along the flow

From 2007 Thomson Higher Education

Upper level driver

Are we satisfied with kinematics so far?

• How does temperature impact cyclone development?

Questions we could not answer:

• How does external and internal heating and impact cyclone development?

500 hPa level 2

Surface level 1

VT = Vg2－ Vg1 =

Thermal wind Balance:

Vorticity:

With certain approximations, we have:

Petterssen’s Development Equation(Carlson (1998))

Vorticity dynamics

vorticity advection at 500 hPa

surface-500 hPa layer-averaged temperature advection

surface-500 hPa layer-averaged adiabatic heating/cooling

surface-500 hPa layer-averaged diabatic heating/cooling

Cyclone Development Equation

Positive Vorticity Advection (PVA)

N

ENegative Vorticity

Positive Vorticity

5x10-5 s-1

10x10-5 s-1

15x10-5 s-1

20x10-5 s-1

Negative Vorticity Advection (NVA)

N

ENegative vorticity

Positive vorticity

4x10-5 s-1

8x10-5 s-1

12x10-5 s-1

16x10-5 s-1

Effects of Vorticity Advection

For a Typical Synoptic Wave:

• Areas of positive (PVA) are often located east of a trough axis

• PVA increases the surface vorticity ζ1 and leads to the formation of a surface low or cyclone

PVA NVA

Trough

Ridge

500 mb

∨

∨

WAA

• Areas with maximum warm (WAA), one has , which leads to an increase in surface vorticity ζ1

and the formation of a surface low or cyclone

Effects of Temperature Advection

• Strong diabatic heating (H >0) always helps to increase surface vorticity ζ1

• Diabatic heating includes radiation, latent heat release from cloud and precipitation, and sensible heat exchange

Effects of Diabatic Heating H

Effects of Adiabatic Heating S• When S < 0, there is whole layer (surface-500 hPa) convergence, which leads to a decrease in surface vorticity and unfavors the development of surface low• Upper level (above 500 hPa) divergence is needed for cyclone development!

Note:

From continuation equation:

We can have:

Therefore:

If there is no surface forced vertical velocity ( ) and the surface-500 pha layer-averaged convergence ( ) leads to , unfavorable to cyclone development.

The surface cyclones intensify due to WAA and an increase in PVA with height

→ rising motion→ surface pressure decreases

With warm air rising to the east of the cyclone, and cold air sinking to the west, potential energy is converted to kinetic energy (baroclinic instability) and the cyclone’s winds become stronger

Surface Cyclone Development

WAA

PVA500mb

Rising

SFC

PressureDecrease

SystemIntensifies

L WAACAA

Surface Cyclone Development

Weather of Extratropic Cyclone

Warm Sector:WarmPotential showers and thunderstorms

Cold Front:Narrow Band of showers and thunderstormsRapid change in wind directionRapid temperature decrease.Rapidly clearing skies behind the front

Occluded Front:Cold with strong windsPrecipitation light to moderateSignificant snow when cold enough

Warm Front:Cloudy and cold.Heavy precipitationPotential sleet and freezing rain

From gsfc.nasa

surface cyclone

Surface weather chart12Z, Wed, Nov 9, 2011

surface cyclone

•Occurred before a trough and after a ridge

advection of + vorticity

500 hPa weather chart12Z, Wed, Nov 9, 2011

How did upper level waves support the developing surface cyclone

advection of warm air

divergence due to curvature

divergence due to deceleration

500 hPa trough

Single synoptic scale cyclone process can cause highly variable surface wind field and impact sea ice

Xiangdong Zhang, IARC

Winter Summer

Climatological characteristics of northern hemispheric cyclone activity

cyclone count/frequency

cyclone central SLP

Winter

Climatological characteristics of northern hemispheric cyclone activity

Summer

• Cyclone is a prominent element of weather system, impacting our daily life.

• Genesis, development, and decay of cyclones result from 3-dimensional, interactive processes between dynamics and thermodynamics.

• Better understanding of cyclones has important implications for improving weather forecast and climate change assessment.

Summary