extratropical cyclones – genesis, development, and decay xiangdong zhang
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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; - PowerPoint PPT PresentationTRANSCRIPT
Extratropical Cyclones – Genesis, Development, and Decay
Xiangdong ZhangInternational Arctic Research Center
Basic FactsExtratropical 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
WAAPVA
500mbRising
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 airdivergence due to curvaturedivergence 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