1. Structure and maintenance of squall lines: A historical overview Robert Fovell UCLA Atmospheric and Oceanic Sciences rfovell@ucla.edu

2. Scope and Objectives Historical overview “Broken lines” of “ordinary cells” having trailing stratiform precipitation Evolution of squall line conceptual models Conceptual models of squall line evolution, structure and behavior

3. Definition of “squall line” Glossary of Meteorology (2000): “a line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of thunderstorms.”

4. Newton and Newton (1959) “[A] squall line generally consist[s] of a large number of thunderstorm cells” with lifetime ~30 min “[C]ontinuous formation of new cells is necessary” created via “successive triggering… by lifting of unstable air over a [rain-produced] ‘pseudo-cold front’”

5. Characteristics Long-lived Unsteady and multicellular Evaporationally-produced subcloud cold pools Cold pool is principal propagation mechanism

6. 8 July 2003, Lincoln, NE

7. A modern conceptual model (e.g., Houze et al. 1989)

8. Squall line vertical x-section

10. Storm-relative flow in storm and far-field; note non-constant shear and upshear tilt

11. Squall line vertical x-section Radar echo envelope

12. Squall line vertical x-section Principal echo features; implied multicellularity

13. Squall line vertical x-section Principal pressure perturbations

14. Conceptual model of a “trailing stratiform” (TS) squall line Houze et al. (1989)

15. Evolution of squall line conceptual models

16. An isolated “ordinary cell” Ludlam (1963)

17. Thunderstorm life cycle The Thunderstorm Project (Braham’s reminiscence) –Aug. 1940: DC-3 crash killed Minnesota senator during storm –1944: Civil Aeronautics Board called for study of storm air motions, after another DC-3 lost lift –Jan. 1945: HR 164 authorized Weather Bureau to study thunderstorm causes, characteristics (didn’t become law) –End of WWII provided the planes and personnel –Project based in Orlando in 1946, Ohio in 1947 (based on storm frequency and military base proximity)

18. Stages of isolated t-storm T-storm Project

19. T-storms not always isolated U = updraft D = downdraft T-storm Project Horizontal cross-section

20. Early models of squall circulation Newton (1963)

21. Early models of squall circulation “[T]he downdraft is drawn as continuous from cloud top to base for the sake of discussion, though there are inadequate observations to verify whether this is typical.” Newton (1963)

22. Newton (1966) Early models of squall circulation “[N]o appreciable portion of the updraft air is likely to descend again to the lower troposphere.”

23. Zipser’s (1977) model (reversed for midlatitude context)

24. Zipser’s (1977) model Transience permits this in 2D (e.g., Rotunno et al. 1988; Fovell and Ogura 1988)

25. Zipser’s (1977) model Inflow layer overturns in “crossover zone”

26. Layer lifting Bryan and Fritsch (2000) “Moist absolutely unstable layer” (MAUL)

27. Pressure perturbations in and near squall lines LeMone et al. (1984) Both buoyancy and dynamic pressure contribute, dominated by former (Fovell and Ogura 1988)

28. Pressure perturbations in and near squall lines Fujita (1963)

29. Mesohigh and wake low Fujita (1955)

30. Mesohigh and wake low Johnson and Hamilton (1988) Fujita (1955)

31. Pre-squall low Hoxit et al. (1977) Pre-squall low ascribed to subsidence warming.

32. Rear inflow current Pandya and Durran (1996)

33. Rear inflow current Colored field: temperature perturbation; Contoured field: horizontal velocity perturbation

34. Rear inflow current

35. Pandya and Durran (1996)

36. Rear inflow current Pandya and Durran (1996)

37. The multicell storm Browning et al. (1976) Four cells at a single time Or a single cell at four times

38. The multicell storm Browning et al. (1976) Fovell and Tan (1998) Unsteadiness represents episodic entrainment owing to local buoyancy-induced circulations.

39. Life cycle of a tropical squall line Leary and Houze (1979)

40. The severe squall line environment From 10 years of severe spring Oklahoma storms Bluestein and Jain (1985)

41. The severe squall line environment

42. Similar in tropical squall lines (below 4 km); e.g., Barnes and Sieckman (1984)

43. Conceptual models of squall line evolution, initiation, and maintenance

44. Some questions (leading to very incomplete answers) How are pre-frontal squall lines initiated? Is a squall line self-maintaining? Why does the storm updraft airflow lean upshear? What determines how strong a storm can be?

45. Cold pool and vertical shear Cold pool and shear are irrelevant Cold pool good, shear bad Cold pool good, shear good Cold pool bad, shear bad, but combination may be good

46. Tepper (1950) “[S]quall lines are propagated pressure jump lines, whose genesis, propagation and destruction are independent of the precipitation which they themselves produce.” “Consequently in following a squall line across the country, it is most important to follow the progress of the pressure jump line, And not… the line of convective activity.”

47. Tepper (1950) (Figure augmented)

48. Newton (1950) “[T]he air above the warm-sector inversion, if one is present, is usually relatively dry and a great amount of lifting would be required…” Cold pools are “insufficient to wholly explain the maintenance of squall-line activity since it is frequently observed that large rain-cooled areas [persist] after squall-line activity dissipates”

49. On shear “It is remarkable that in spite of the marked vertical wind shears associated with squall-storms, they are long-lived, often travelling long distances at rather uniform speed” Ludlam (1963)

50. A role of strong shear? Newton and Newton (1959)

51. “Upshear” tilt Ludlam (1963), via Rotunno et al. (1988)

52. Early numerical experiments

53. Hane (1973) 2D model initialized with moderate shear

54. Hane (1973)

62. “[T]he system, rather than reaching a quasi-steady state, undergoes a series of developments…” owing to the “adverse effects” of 2D

63. Hane (1973) [T]he squall line thunderstorm, once initiated, maintains itself” …as long as it remains in a favorable environment.

64. Discussion of Hane (1973) Convection strong prior to cold pool development Storm weaker, more intermittent after pool appearance Upshear tilt

65. Thorpe et al. (1983) 2D model

66. Thorpe et al. (1983) Steadiest storm, most precipitation (amount and intensity)

67. Thorpe et al. (1983) This nearly steady storm “required strong low-level shear to prevent the upstream gust front from propagating rapidly away from the storm.”

68. Thorpe et al. (1983) This nearly steady storm “required strong low-level shear to prevent the upstream gust front from propagating rapidly away from the storm.”

69. RKW theory Rotunno et al. (1988) “Cold pool bad, shear bad, But combination may be good.”

70. My take on RKW theory: Cold pool not an unalloyed good; lifting comes at a price

71. A garden-variety multicell storm

72. Cold pool experiment (‘no-cloud cloud model’) Control run Deactivate evaporation cooling

73. Clouds without cold pool lifting Crook and Moncrieff (1988)

75. Cell initiation by trapped gravity waves Fovell et al. (2006)

76. Summary Historical overview (incomplete) Modern conceptual model of a TS squall line Evolution of squall line conceptual models Conceptual models of squall line evolution

77. end

78. Newton & Newton (1959)

79. Other squall-type configurations Parker and Johnson (2000)