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Cyclogenesis and Cyclolysis

Cyclogenesis and Cyclolysis. ATS 113. What We’ll Learn:. The Polar Front causes the Midlatitude Jet Stream (by the Thermal Wind Relationship). The MLJ has troughs and ridges. Troughs and ridges lead to patterns of divergence aloft, causing low pressure at the surface. Midlatitude Jet Stream.

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Cyclogenesis and Cyclolysis

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  1. Cyclogenesis and Cyclolysis ATS 113

  2. What We’ll Learn: • The Polar Front causes the Midlatitude Jet Stream (by the Thermal Wind Relationship). • The MLJ has troughs and ridges. • Troughs and ridges lead to patterns of divergence aloft, causing low pressure at the surface.

  3. Midlatitude Jet Stream

  4. Was found directly above the Polar Front due to the Thermal Wind Relationship. 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

  5. “Zonal”

  6. “Meridional”

  7. Either way, the POLAR FRONT is directly beneath the jet.

  8. “Troughs” and “Ridges”

  9. L Vorticity • The “spin of the wind”. • Defined so that CYCLONIC is POSITIVE.

  10. The trough has “positive vorticity”:

  11. The ridge has “negative vorticity”:

  12. Between has no vorticity.

  13. Between has “positive vorticity advection”! PVA

  14. NVA ahead of ridge, PVA ahead of trough PVA NVA

  15. What is PVA? • The process by which the winds are bringing air that has higher vorticity values. Location A has positive vorticity. Location B has no vorticity but does have “positive vorticity advection”. B A

  16. What is PVA? • As air moves from location A to location B, it needs to lose its spin. B A

  17. Losing spin • How does a spinning figure skater lose his spin?

  18. Losing spin • How does a spinning figure skater lose his spin? • Spreading his arms!

  19. So… • Spreading out is a way of losing spin.

  20. A region with PVA is a location in which the air must lose its spin! PVA

  21. DIVERGENCE!

  22. Down at the surface…

  23. …upper level divergence… divergence

  24. …means low pressure at the surface! divergence L

  25. Notice where the low is: East of the trough West of the ridge In the region of positive vorticity advection! divergence L

  26. Notice where the low is: East of the trough West of the ridge In the region of positive vorticity advection! L

  27. Notice where the low is: L East of the trough West of the ridge In the region of positive vorticity advection!

  28. Why was the midlatitude jet here in the first place? POLAR FRONT!

  29. Why was the midlatitude jet here in the first place? POLAR FRONT!

  30. “Cyclogenesis” creates a low along this front at the surface: L

  31. Life Cycle of a Midlatitude Cyclone! L

  32. Low eventually becomes “occluded”… L

  33. …no longer a temperature gradient near the low itself… L

  34. …by Thermal Wind Relationship, no longer a reason for the jet to be here! L

  35. Jet will move to be above the polar fronts! L

  36. Now what kind of vorticity advection is above the low? NONE! L

  37. So how much upper level divergence is above the low? NONE! L

  38. No “support” for the low– it begins to die… CYCLOLYSIS! L

  39. Birth and Death • Cyclogenesis: • Strengthening a cyclone by lowering its pressure. • Happens when PVA in the jet stream causes upper level divergence. • Cyclolysis: • Weakening a cyclone by increasing its pressure. • Happens when the jet stream is no longer directly above the low due to the formation of an occluded front.

  40. Real Life Examples: Jet stream has a very sharp trough in the midwest.

  41. Real Life Examples: Positive vorticity in trough; negative vorticity in ridge.

  42. Real Life Examples: Positive Vorticity Advection near Detroit

  43. Real Life Examples: A strong low near Detroit!

  44. Another Example: A huge trough in the Rockies.

  45. Another Example: Positive Vorticity in the trough.

  46. Another Example: Positive Vorticity Advection ahead of the trough.

  47. Another Example: L A low and precipitation at the surface!

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