Extratropical Cyclones – Genesis, Development, and Decay Xiangdong Zhang

1. Extratropical Cyclones – Genesis, Development, and Decay Xiangdong Zhang International Arctic Research Center

3. Basic Facts • Extratropical cyclones is amajor 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.

4. 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

5. Bergen Cyclone Model (BCM)

6. Mechanism of cyclone development: Baroclinic instability Center of Gravity Z h Warm cold Unstable Stable Baroclinic Instability: Available potential energy (APE)  kinetic energy (air movement -> wind) h ≈ 0 Unstable Stable

7. 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?

8. 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

9. Surface – upper troposphere coupling • Geostrophic wind: the wind when it is in perfect geostrophic balance: • Ageostrophic wind: difference between the actual wind and the wind when it is in perfect geostrophic balance:

10. Force Balance Free Atmosphere component component <0: cyclonic curving Ageostrophic wind: >0: anticyclonic curving

11. Ageostrophic wind when the air curves cyclonically: Sub-geostrophic wind: slower than the geostrophic wind. Low Pressure • The centripetal acceleration breaks the geostrophic balance; Pressure Gradient Force Centripetal Acceleration • The ageostrophic wind points the opposite direction of the geostrophic wind. Coriolis Force High Pressure

12. Ageostrophic wind when the air curves anticyclonically: Super-geostrophic wind: faster than the geostrophic wind. High Pressure • The centripetal acceleration breaks the geostrophic balance; Coriolis Force Centripetal Acceleration • The ageostrophic wind points the same direction of the geostrophic wind. Pressure Gradient Force Low Pressure

13. Ageostrophic wind when the air speeds up: Ageostrophic wind when the air slows down: Low Pressure • The pressure gradient increases and air blows toward lower pressure side; Pressure Gradient Force • The ageostrophic wind points the left of the geostrophic wind. Coriolis Force High Pressure • Opposite.

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

15. Summary II: Effects from varying pressure gradients along the flow Low Pressure Divergence Convergence Pressure Gradient Force CF > PGF (PGF decrease) PGF > CFP (PGF increases) Coriolis Force Convergence Divergence High Pressure new old

17. Upper level driver From 2007 Thomson Higher Education

18. 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?

19. Vorticity dynamics Thermal wind Balance: VT = Vg2－Vg1 = Vorticity: 500 hPa level 2 Surface level 1 With certain approximations, we have: Petterssen’s Development Equation (Carlson (1998))

20. Cyclone Development Equation 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

21. 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

22. 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

23. Effects of Vorticity Advection Ridge • 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 500 mb Trough PVA NVA ∨ ∨

24. Effects of Temperature Advection • 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 WAA

25. Effects of Diabatic Heating H • 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 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.

26. SurfaceCyclone 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 WAA L Rising CAA 500mb PVA WAA SFC Pressure Decrease System Intensifies

27. SurfaceCyclone Development

28. Weather of ExtratropicCyclone • Warm Front: • Cloudy and cold. • Heavy precipitation • Potential sleet and freezing rain • Occluded Front: • Cold with strong winds • Precipitation light to moderate • Significant snow when cold enough From gsfc.nasa • Cold Front: • Narrow Band of showers and thunderstorms • Rapid change in wind direction • Rapid temperature decrease. • Rapidly clearing skies behind the front • Warm Sector: • Warm • Potential showers and thunderstorms

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

31. 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

32. Single synoptic scale cyclone process can cause highly variable surface wind field and impact sea ice Xiangdong Zhang, IARC

33. Climatological characteristics of northern hemispheric cyclone activity Winter Summer cyclone count/frequency

34. Climatological characteristics of northern hemispheric cyclone activity Winter Summer cyclone central SLP

35. 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.