Transcript Extratropical Cyclones
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.
Outline
Goal: Understand cyclone from simple model to complex dynamics
•
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 Z Warm cold Unstable Stable
Baroclinic Instability: Available potential energy (APE) (air movement -> wind) kinetic energy
Center of Gravity h h ≈ 0 Unstable Stable
Are we satisfied with BCM so far?
Questions we could not answer:
•
How do upper level waves disturb the surface cyclone formation?
•
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
•
Geostrophic wind: the wind when it is in perfect geostrophic balance:
f k
´
v g
= 1 r ´
p
•
Ageostrophic wind: difference between the actual wind and the wind when it is in perfect geostrophic balance:
a
=
v
-
v g
d
V
d
t
= 1 r d d
p s s
+ ( -
fV
1 r d d
p n
)
n
Force Balance Free Atmosphere
s t
+ 1 d
s
dq
s t
d
V
d
t
= d (
Vs
) d
t
= d
V
d
t s
+
V
d
s
d
t
d
V
d
t s
= 1 r d
p
d
s s s
component
d
S s t
d
s
=
s
dq d
s s
= dq d
s s
dq d
s
d
t
= dq d
t
d d
s s V
d
s
d
t
=
V V R n
= ( -
fV
= 1 r d
S
d
t R
d d
p n
)
n
d
s
d
s n
component
= d
S
d
t R
-
fV
1 r d
s
d
s
d
p
d
n
-
V
2
R
Ageostrophic wind:
V a
= -
V
2
fR
d
s
d
t
=
R V R n
= -
fV g
-
n
( d
t
® 0)
fV a
1 r d
p
d
n
-
V
2
R
= 0
<0: cyclonic curving >0: anticyclonic curving
Ageostrophic wind when the air curves cyclonically:
•
The centripetal acceleration breaks the geostrophic balance;
v t
1 •
The ageostrophic wind points the opposite direction of the geostrophic wind.
v a
Low Pressure Pressure Gradient Force Centripetal Acceleration
v t v t
+ 1
V a
= -
V
2
fR
< 0
Coriolis Force High Pressure Sub-geostrophic wind: slower than the geostrophic wind.
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.
V a
= -
V
2
fR
> 0
High Pressure
v t
+ 1
Coriolis Force Centripetal Acceleration
v t v a
Pressure Gradient Force Low Pressure
v t
1
Super-geostrophic wind: faster than the geostrophic wind.
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.
Low Pressure Pressure Gradient Force
v t
1
Coriolis Force
v t
High Pressure Ageostrophic wind when the air slows down:
•
Opposite.
v a
Summary I: Curvature effects (uniform pressure gradients along the flow)
Summary II: Effects from varying pressure gradients along the flow Low Pressure Convergence Pressure Gradient Force Divergence PGF > CFP (PGF increases) CF > PGF (PGF decrease) Convergence Coriolis Force Divergence High Pressure old new
Upper level driver
From 2007 Thomson Higher Education
Are we satisfied with kinematics so far?
Questions we could not answer:
•
How does temperature impact cyclone development?
•
How does external and internal heating and impact cyclone development?
Vorticity dynamics
Thermal wind Balance:
V
T
= V
g2 －
V
g1
=
Vorticity: z
g
1 = z
g
2 z
T
¶z 1 ¶
t
= ¶z 2 ¶
t
¶z
T
¶
t
With certain approximations, we have: 500 hPa level 2 Surface level 1 Petterssen ’ s Development Equation (Carlson (1998))
Cyclone Development Equation
A
z 2
vorticity advection at 500 hPa
A T
surface-500 hPa layer-averaged temperature advection
S
surface-500 hPa layer-averaged adiabatic heating/cooling
H
surface-500 hPa layer-averaged diabatic heating/cooling
Positive Vorticity Advection (PVA)
N
Negative Vorticity
E 5x10 -5 s -1 10x10 -5 s -1 15x10 -5 s -1 20x10 -5 s -1
Positive Vorticity
Negative Vorticity Advection (NVA)
N
Negative vorticity
E 4x10 -5 s -1 8x10 -5 s -1 12x10 -5 s -1 16x10 -5 s -1
Positive vorticity
Effects of Vorticity Advection
A
z 2
For a Typical Synoptic Wave:
• Areas of positive ( 2
) trough axis
•
PVA vorticity ζ 1 and leads to the formation of a surface low or cyclone Trough PVA Ridge NVA 500 mb
Effects of Temperature Advection
A T
• Areas with maximum warm (
T
WAA ), one has , which leads to an increase in surface vorticity ζ and the formation of a surface low or cyclone 1 WAA
Effects of Diabatic Heating H
• 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 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: ¶w ¶
p
w = = ( ¶
u
¶
x
+ w
ps
¶ ¶
v y
) = -
D
* (
p
-
D p s
) Therefore: If there is no surface forced vertical velocity ( ) and the surface-500 pha layer-averaged
s
= 0
D
< 0
S
< 0
S
= w
p s
( g
d
g ) -
D
(
p
-
p s
)( g
d
g )
Surface Cyclone Development
The surface cyclones intensify due to WAA and an increase in PVA with height → rising motion → surface pressure decreases
500mb
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
SFC Rising PVA WAA Pressure Decrease System Intensifies
Surface Cyclone Development
Weather of Extratropic Cyclone
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 Cold Front:
Narrow Band of showers and thunderstorms
Rapid change in wind direction
Rapid temperature decrease.
Rapidly clearing skies behind the front
From gsfc.nasa
Warm Sector:
Warm
Potential showers and thunderstorms
Surface weather chart 12Z, Wed, Nov 9, 2011 surface cyclone
500 hPa weather chart 12Z, Wed, Nov 9, 2011 How did upper level waves support the developing surface cyclone
•
Occurred before a trough and after a ridge advection of + vorticity advection of warm air divergence due to curvature divergence due to deceleration 500 hPa trough surface cyclone
Single synoptic scale cyclone process can cause highly variable surface wind field and impact sea ice
Xiangdong Zhang, IARC
Climatological characteristics of northern hemispheric cyclone activity Winter Summer cyclone count/frequency
Climatological characteristics of northern hemispheric cyclone activity Winter Summer cyclone central SLP
Summary
•
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.