Extratropical Cyclones - IARC Research

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Oct 27, 2013 (3 years and 7 months ago)

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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


S
urface


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 of

Gravity

h

h
≈ 0

Baroclinic

Instability:
Available
potential energy (APE)


kinetic
energy

(air movement
-
> wind)


Warm



cold

Z

Unstable

S
table

Unstable

S
table

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?

p
lanetary 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
:


G
eostrophic

wind
:

the

wind

when

it

is

in

perfect

geostrophic

balance
:

Force Balance

Free Atmosphere

c
omponent

c
omponent

Ageostrophic

wind:

<0: cyclonic curving

>
0: anticyclonic curving

Ageostrophic

wind when the air curves cyclonically:


The
c
entripetal
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
:

E
ffects

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

V
T

= V
g2

V
g1

=


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

E

Negative Vorticity

Positive Vorticity

5x10
-
5

s
-
1

10x10
-
5

s
-
1

15x10
-
5

s
-
1

20x10
-
5

s
-
1

Negative Vorticity Advection (NVA)

N

E

Negative 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 ζ
1
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 ζ
1


and the formation of a surface low


or cyclone



Effects of Temperature Advection



Strong diabatic heating (H >0) always helps to increase surface vorticity ζ
1



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

PVA

500mb

Rising

SFC

Pressure

Decrease


System

Intensifies

Surface
Cyclone Development

Weather of
Extratropic

Cyclone

Warm Sector:


Warm


Potential showers and thunderstorms

Cold Front:


Narrow Band of showers and
thunderstorms


Rapid change in wind direction


Rapid temperature decrease.


Rapidly clearing skies behind the front

Occluded Front:


Cold with strong winds


Precipitation light to moderate


Significant snow when cold enough

Warm Front:


Cloudy and cold.


Heavy precipitation


Potential sleet and freezing rain

From gsfc.nasa

s
urface cyclone

Surface weather chart

12Z, Wed, Nov 9, 2011

s
urface cyclone


Occurred before a trough
and after a ridge

advection of + vorticity

500
hPa

weather chart

12Z, Wed, Nov 9, 2011

How did upper level waves
support the developing
surface cyclone

advection of warm air

d
ivergence due to curvature

d
ivergence 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

c
yclone count/frequency

c
yclone 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