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Convective initiation ahead of squall lines

Convective initiation ahead of squall lines. Robert Fovell UCLA Atmospheric & Oceanic Sciences rfovell@ucla.edu. (Fovell, Mullendore and Kim 2006, MWR). Radar image of a squall line. Vertical cross-section. Vertical cross-section. Vertical cross-section.

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Convective initiation ahead of squall lines

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  1. Convective initiation ahead of squall lines Robert Fovell UCLA Atmospheric & Oceanic Sciences rfovell@ucla.edu (Fovell, Mullendore and Kim 2006, MWR)

  2. Radar image of a squall line

  3. Vertical cross-section

  4. Vertical cross-section

  5. Vertical cross-section

  6. A typical multicellular squall line

  7. Vertical cross-section “discrete convective initiation”

  8. Vertical cross-section

  9. Vertical cross-section X “discrete propagation”

  10. 7 May 1995, early evening

  11. 0509Z - Hastings, NE radar8 July 2003 gust front

  12. 0539Z - Hastings, NE radar new cells ~ 18 km ahead

  13. 0549Z - Hastings, NE radar

  14. 0609Z - Hastings, NE radar

  15. Animation of Hastings radar

  16. 05 June 2004 X = Hays, KS

  17. 05 June 2004 X = Hays, KS

  18. 21 June 2003, W Oklahoma ~ after midnight

  19. 2245Z (545 PM CDT)

  20. 2245Z (545 PM CDT) Ft. Worth

  21. 00Z Fort Worth hodograph

  22. Rolls in an ARPS simulation

  23. How do afternoon roll cloudsinfluence nocturnal convection? By organizing the moisture field; effect survives rolls themselves

  24. MM5 simulation • 4 km horizontal resolution; 250x330 pts • Start 12Z previous day • Initial/boundary conditions from Eta model • MRF PBL scheme

  25. MM5 model animation • 3 hour animation (01-04Z) • Colored field is 2 m water vapor • Vertically integrated condensate contoured • 10 m wind vectors

  26. MM5 moisture bands • Remains of convective rolls present in model on previous afternoon • Rolls are spurious • reflect deficiency of PBL scheme • ~40 km wavelength >> theoretical value • actual roll clouds ~ theoretical value • Rolls are fortuitous • suggest orientation for the new cell lines

  27. “Action at a distance” mechanism Trapped internal gravity waves

  28. An ARPS simulation • 2D & 3D models • Horizontally homogeneous initial conditions • ∆x = 1 km, ∆z ≥ 40 m • Warm rain processes • Starts late afternoon

  29. Vertical velocity (colored) ~ sunrise main updraft cold pool

  30. Vertical velocity (colored) ~ sunrise gravity waves 20 m/s

  31. Vertical velocity (colored) ~ sunrise Trapping or ducting below 8-9 km

  32. Vertical velocity (colored) ~ sunrise

  33. Gravity wave ducting z x Scorer parameter

  34. Closer look at Scorer parameter In mountain wave derivation, we had Difference: mountain waves presumed steady, therefore  = 0 and c = /k = 0. Also, N*2 is BV frequency modified for moisture.

  35. Ducting: sharp decrease of l2 with height • Here c > U • Forward anvil as wave duct • decrease in ambient stability • anvil: warming below, cooling above; saturated • partially opposed by (U - c) decrease • jet-like wind profile - curvature shear

  36. upstream sounding

  37. Uzz min upstream sounding

  38. upstream sounding

  39. Trapped waves leading to discrete initiation

  40. 6 h ARPS model animation

  41. Discrete initiation by gravity waves alone Note forward anvil

  42. Discrete initiation by gravity waves alone Gravity waves trapped beneath anvil

  43. Discrete initiation by gravity waves alone Wave-relative flow shown (recall c > U)

  44. Transient trapping conditions

  45. Internal gravity waves alone apparently can’t account for the orientation of the new cell bands Combine gravity waves & moisture bands

  46. Hypothesis Plane view, looking from above Moisture bands remaining from earlier roll activity

  47. Hypothesis Squall line and its forward anvil

  48. Hypothesis Trapped internal gravity waves beneath anvil

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