1. GEU 0047: Meteorology Lecture 12 Mid-latitude Cyclones

2. Vorticity

3. Winds Aloft

4. Divergence Aloft and Cyclogenesis Initially No Upper Divergence No Lower Divergence Trough Approaches from the West Upper Divergence forms due to the trough No Lower Divergence

5. Divergence Aloft and Cyclogenesis Trough Approaches from the West No Lower Divergence The divergence aloft initiates an upward vertical motion beneath the upper level divergence.

6. Divergence Aloft and Cyclogenesis Surface convergence develops in response to the rising motion.

7. Divergence Aloft and Cyclogenesis If more mass is being removed from the column by the upper level divergence than is replaced by surface convergence, then a low pressure center at the surface will either develop or deepen. Cyclogenesis!!!!! L

8. Formation of Cyclones Cyclogenesis -- The development or strengthening of a region of low pressure -- a cyclone. Conditions: –Cyclonic flow must be established at the surface. Cyclonic flow is CCW in the Northern Hemisphere. –Inward flow of air at the surface must be supported by outward flow of air aloft. This is the “upper level support.”

9. Ocean Wave Development Undisturbed Water Friction causes waves to develop at the surface of the water.

10. Wave Development If the wind is strong enough, the wave can crest and “break.”

11. Now, consider the atmosphere: H cP mT Wave Cyclone Development H

12. The cyclone may deepen. cP mT Wave Cyclone Development L The warm air moves northward and the cold air moves southward. If the pressure falls, the cyclone deepens or gets more intense.

14. Cyclone Fronts

15. Warm Front

16. Cold Front

17. Cyclone Stages

19. A cloud formation in the shape of a “comma” is characteristic of a well developed wave cyclone. This characteristic shape is often used to find storms over the oceans using satellite imagery. The cyclone matures. cP mT Wave Cyclone Development L Cloud “Shield”

20. Wave Cyclone “Comma”

21. Satellite Image © 1997 Prentice-Hall, Inc. -- From: Moran and Morgan, Meteorology Comma Cloud Dry Slot L Cold Front Warm Front

22. The cold front overtakes the warm front and lifts the warm air off the ground. The cold air now surrounds the cyclone and, since the sloping temperature gradient (front) is gone, the pressure gradient weakens and the low slowly dies. The cyclone occludes. cP mT Wave Cyclone Development L

23. Extratropical Cyclone Surface Map © 1997 Prentice-Hall, Inc. -- From: Moran and Morgan, Meteorology

24. Cyclone Evolution

25. Occlusion

28. L 4 3 2 1 Weather Associated With A Wave Cyclone è1. Northwest Sector èUsually cold with strong and gusty winds out of the northwest. èPrecipitation light to moderate, with heavier precipitation closer to the low. èIf cold enough, snowfall can be significant.

29. L 4 3 2 1 Weather Associated With A Wave Cyclone è3. Warm Front (South Side) èUsually fair and warm. èLater in the day, showers and thunderstorms may develop. èSome thunderstorms may be heavy but most are of the thundershower type.

30. L 4 3 2 1 Weather Associated With A Wave Cyclone è4. Warm Front (North side) èMostly cloudy and cold. èPrecipitation often heavy and occasionally substantial. èWinds often out of the east or northeast. èDepending on the temperature sounding, potential for sleet and freezing rain.

31. Cyclone Weather Table I II IV VI V VIII VII III

32. Cyclone Weather Table I In between cyclones and anticyclones or High pressure approaching. Stable Air, Clearing conditions. Clear Skies likely for many days.

33. Cyclone Weather Table II The passage of a cold front. The approach of an anticyclone. Cyclone passes West to East, NorthEast of your location.

34. Cyclone Weather Table III Cyclone passes from directly North of you, and moves off to the East.

35. Cyclone Weather Table IV The approach of an anticyclone. Cyclone approaching your location from the SouthWest to the North.

36. Cyclone Weather Table V The approach of an anticyclone. Cyclone sliding from NorthWest to SouthEast of your location.

37. Cyclone Weather Table VI Cyclone approaching from NorthWest of your location.

38. Cyclone Weather Table VII Cyclone sliding from North to South, West of your location.

39. Cyclone Weather Table VIII Cyclone approaching from North of your location.

40. Cyclone Weather

41. Cyclone Weather Table I II IV VI V VIII VII III

42. Idealized sketch of the thermally direct circulation in the zonal (X-Z) plane for a developing baroclinic wave in mid-latitude. Solid lines: pressure surfaces Dashed lines: vertical tilt of the trough and ridge axes Heavy lines: position of tropopause Arrows: flows