Cyclogenesis in Polar Airstreams

1. Cyclogenesis in Polar Airstreams • Not all midlatitude cyclones develop along the main polar front/baroclinic zone. • Range in size from comma clouds (big) to polar lows (small)

2. Comma Clouds: 500-1500 km

3. Polar Lows: 100-500 km

4. Comma Clouds • Convection and sometimes stratiform-type clouds organized into a comma-shaped patterns • Smaller than normal synoptic systems and on the cold side of the midlatitude jet stream. • Most apparent over the oceans during a period with high-amplitude, long-wave trough development

5. Generally of a smaller scale (500 to 1000 km) than classic midlatitude cyclones.

6. Frequently multiple with typical spacing of 1000-1500 km

7. Usually associated with the region of positive vorticity advection (PVA) associated with a short-wave trough aloft.

8. Scale often grows in time, particularly as they move through a long wave trough

9. Three Stages of Comma Cloud Development • Incipient Stage • Two troughs: large scale one and other associated with developing comma • Appreciable baroclinicity with comma • Intensifying Stage • Convective elements grow in size and merge. • Size of system increases • Low center may appear • Stronger advections and front-like characteristics • Mature Stage • Large size and movement to the forward side of the long-wave trough. • Difficult to differentiate from a normal polar front cyclone

10. Associated with regions of appreciable baroclinicity (temperature gradient) on the cold side of a major baroclinic zone (“polar front”).

11. Often develop in conditionally unstable environments with lots of convection

12. Comma Clouds • Most apparent over oceans in winter, but can develop over land

13. 1445Z/05 GOES-12 Visible 2 1

14. 1745Z/05 GOES-12 Visible 2 1

15. 2045Z/05 GOES-12 Visible 2 1

16. 15Z/05 2 2

17. Some, But Not All, Associated with Lightning Strikes Jan 18-21 2010 Lightning in Yellow Nov 15-17 2009

18. Sea-level low-pressure center is sometimes found under the comma head, with a trough of low pressure under the trailing edge of the comma tail.

19. Sometimes the associated trough can develop frontal characteristics

20. There are a variety of ways for comma clouds, and their associated vorticity maxima, to interact with the main baroclinic zone/polar front

21. Little Interaction

22. “Instant” Occlusion

23. Instant Occlusion • The comma cloud/PVA maximum can excite the development of a wave on a preexisting front. • The comma cloud combines with the developing wave to form what appears to be a more occlusion WITHOUT the usual occluded front evolution.

24. Why such a small scale? • Baroclinicity, latent heat release, and low stability appear to accompany most comma clouds. • Several studies (e.g., Gall 76 and Staley and Gall 76) suggest that baroclinic instability in concert with low stability in the lower troposphere could contribute to such small scales. • Needs more work.

25. Polar Lows Look somewhat like hurricanes—spiral rain bands, cloud free eye

26. Polar Lows (also known as Arctic Hurricanes!) • Small scale: typically 300 to 800 km in size • Usually develop near the ice margin where relatively warm, open water is adjacent to ice fields or cold continents. • Thus, they develop in a region of very strong, low-level atmospheric baroclinicity. • Low-stability environment as cold air moves over warm water. Usually convective clouds are present, frequently in linear, cloud streets.

27. Polar Lows • Form rapidly when short-wave troughs aloft approach such baroclinic, unstable regions. • Favored locations: Bering Sea, Greenland, Norwegian and Barents Seas, Gulf of Alaska.

28. Polar Lows

29. Mesoscale Structure

33. Polar Low Mechanisms • Because they look like hurricanes, some have suggested they grow by similar air-sea interaction mechanisms: • CISK (Conditional Instability of the Second Kind) • WISHE (Wind Induced Surface Heat Exchange) • Others have suggested that baroclinic instability in the presence of low stability is dominant. • Probably both mechanisms are important.

34. The END