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Air masses and fronts

Air masses and fronts. Textbook, Chapter 9. Discussion. How do precipitation systems form?. How does this precipitation form?. How does this precipitation form?. mountains. How does this precip form?. How does this storm form?. let’s revisit the lifting mechanisms …. Five Mechanisms.

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Air masses and fronts

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  1. Air masses and fronts Textbook, Chapter 9

  2. Discussion How do precipitation systems form?

  3. How does this precipitation form?

  4. How does this precipitation form? mountains

  5. How does this precip form?

  6. How does this storm form? let’s revisit the lifting mechanisms …

  7. Five Mechanisms 1. Convective ascent spontaneous buoyant Cumulus clouds …

  8. The 4 other mechanisms are forced ascent …(a) over a mountain (b) or a front

  9. (c) Convergence into surface low

  10. (d) Uplift forcedby jet stream Upper-level divergence

  11. Areas of jet-stream driven ascent

  12. Cloud & precipitation formation mechanisms • Buoyant ascent [bubble ascent] • Forced ascent [layer ascent] • Orographic • Frontal • Low-level convergence (friction) • Upper-level divergence (jet stream) • Many clouds are the result of several simultaneous processes.

  13. 1. Buoyant ascent

  14. 1. Buoyant ascent • We’ve talked about stability and parcel ascent (Chapter 4, Lab 7). • Remember that in a conditionally unstable atmosphere, a cloud will continue to rise as long as its core temperature exceeds that of the surroundings

  15. cumulus growth stable environmental lapse rate stable condi- tionally unstable stable conditionally unstable moist dry Cumulus humilis Cumulus congestus Cumulonimbus

  16. Lifting up a mountain may cause destabilization, …which is why Cu clouds first form there

  17. Cloud & precipitation formation mechanisms • Buoyant ascent [bubble ascent] • Forced ascent [layer ascent] • Orographic • Frontal • Low-level convergence (friction) • Upper-level divergence (jet stream) • Many clouds are the result of several simultaneous processes.

  18. (a)Orographic clouds Altocumulus lenticularis, Greenland

  19. The Foehn or Chinook effect Quantifying the Foehn effect (web link)

  20. Chinook wall cloud

  21. Pop quiz: when a good westerly wind blows and heavy snowfall occurs over the Cascades, then in Yakima on the east side it will be ……. than in Seattle on the west side. A: sunnier but cooler B: sunnier, warmer, drier, and windier C: sunnier, cooler, more damp, and calmer D: cloudier, cooler, more damp, and windier. Yakima Seattle

  22. Cloud & precipitation formation mechanisms • Buoyant ascent [bubble ascent] • Forced ascent [layer ascent] • Orographic • Frontal • Low-level convergence (friction) • Upper-level divergence (jet stream)

  23. (b) Uplift along fronts A front is a sudden transition between airmasses

  24. Fronts separate airmasses

  25. cP cT mP mT continental maritime Types of airmasses polar tropical

  26. Fig. 9.3

  27. Fig. 9.4

  28. Air masses have distinct lapse rates, and therefore also distinct cloud and precipitation characteristics

  29. Pop quiz: what airmass is this ? cP cT mP mT

  30. cP cT mP mT Pop quiz: what airmass is this ?

  31. Pop quiz: what airmass is this ? cP cT mP mT Hint: this happens in winter

  32. Warm fronts The cold air recedes …

  33. Warm front clouds Where is the cold air??

  34. Fig. 9.16

  35. Cold fronts

  36. Cold front clouds • stable air ahead of cold front light rain or dry (2) unstable air ahead of cold front

  37. Fig. 9.14

  38. Occluded fronts Stage I Stage II

  39. Cross section through an occluded front

  40. Cold occlusion (eastern US) Warm occlusion (western US) Fig. 9.18

  41. Stationary fronts Current analysis

  42. 3D view of fronts during the evolution of a frontal disturbance

  43. drylines Thunderstorms, sometimes severe ones, can be triggered along the dryline.

  44. Case study: winter storm of 10-13 Dec 2000

  45. Cloud & precipitation formation mechanisms • Buoyant ascent [bubble ascent] • Forced ascent [layer ascent] • Orographic • Frontal • Low-level convergence (friction) • Upper-level divergence (jet stream)

  46. (c) Low-level convergence into lows or trofs • Tropical lows (hurricanes) • extract their energy from latent heat release • Mid-latitude lows • extract most of their energy from the jet stream

  47. Tropical cyclogenesis LOW-LEVEL CONVERGENCE INTO A TROF

  48. Cross section thru a tropical cyclone LOW-LEVEL CONVERGENCE

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