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 km in diameter;  Intense: central pressure ranging from 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 of Gravity h h ≈ 0 Baroclinic Instability: Available potential energy (APE)  kinetic energy (air movement -> wind) Warmcold Z UnstableStable UnstableStable

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 Balance Free Atmosphere component Ageostrophic 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 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 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 PVANVA 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 L WAA CAA

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

surface cyclone Surface weather chart 12Z, Wed, Nov 9, 2011

surface 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 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

WinterSummer 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