2. 1 Supercell Thunderstorms Adapted from Materials by Dr. Frank Gallagher III and Dr. Kelvin Droegemeier School of Meteorology University of Oklahoma Part I

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5. 4 Supercell Thunderstorms n A very large storm with one principal updraft n Quasi-steady in physical structure – –Continuous updraft – –Continuous downdraft – –Persistent updraft/downdraft couplet n Rotating Updraft --- Mesocyclone n Lifetime of several hours n Highly three-dimensional in structure

6. 5 Supercell Thunderstorms n Potentially the most dangerous of all the convective types of storms n Potpourri of severe and dangerous weather – –High winds – –Large and damaging hail – –Frequent lightning – –Large and long-lived tornadoes

7. 6 Supercell Thunderstorms n Form in an environment of strong winds and high shear – –Provides a mechanism for separating the updraft and downdraft

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9. 8 Structure of a Supercell Storm Updraft Downdraft

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11. 10 Supercell Thunderstorms n Initial storm development is essentially identical to the single cell thunderstorm – –Conditional instability – –Source of lift and vertical motion – –Warm, moist air

12. 11 Schematic Diagram of a Supercell Storm (C. Doswell)

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14. 13 Structure of a Supercell Storm Mesocyclone

15. 14 Supercell Structure Inflow © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems Mesocyclone Forward Flank Downdraft Rear Flank Downdraft Flanking Line/ Gust Front Tornado Gustnado

16. 15 A Supercell on NEXRAD Doppler Radar Hook Echo

17. 16 A Supercell on NEXRAD Doppler Radar Hook Echo

18. 17 Where is the Supercell?

19. 18 Where is the Supercell?

20. 19 Supercell Types n Classic n Low-precipitation n High-precipitation

21. 20 Low Precipitation (LP) Supercells n Little or no visible precipitation n Clearly show rotation n Cloud base is easily seen and is often small in diameter n Radar may not indicate rotation in the storm although they may have a persistent rotation n LP storms are frequently non-tornadic n LP storms are frequently non-severe

22. 21 LP Supercell © 1993 American Geophysical Union -- From: Church et al., The Tornado Side View Schematic

23. 22 LP Supercell © 1993 American Geophysical Union -- From: Church et al., The Tornado Top View Schematic

24. 23 LP Supercell © 1995 Robert Prentice

25. 24 LP Supercell © 1995 Robert Prentice

26. 25 Another LP Supercell © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

27. 26 A Tornadic LP Supercell © 1998 Prentice-Hall, Inc. -- From: Lutgens and Tarbuck, The Atmosphere, 7 th Ed. 26 May 1994 -- Texas Panhandle

28. 27 High Precipitation (HP) Supercells n Substantial precipitation in mesocyclone n May have a recognizable hook echo on radar (many do not, however) n Reflectivities in the hook are comparable to those in the core n Most common form of supercell n May produce torrential, flood-producing rain n Visible sign of rotation may be difficult to detect -- Easily detected by radar

29. 28 HP Supercells © 1993 American Geophysical Union -- From: Church et al., The Tornado

30. 29 HP Supercells © 1993 American Geophysical Union -- From: Church et al., The Tornado

31. 30 HP Supercell Heaviest Precipitation (core) 4 OCT 1998 2120 UTC KTLX Kansas Oklahoma Woods County, Oklahoma

32. 31 HP Supercell Heaviest Precipitation (core) 4 OCT 1998 2150 UTC KTLX Kansas Oklahoma Developing Cells Twenty minutes later …..

33. 32 Classic Supercells n Traditional conceptual model of supercells n Usually some precipitation but not usually torrential n Reflectivities in the hook are usually less than those in the core n Rotation is usually seen both visually and on radar

34. 33 Classic Supercells © 1993 American Geophysical Union -- From: Church et al., The Tornado

35. 34 Classic Supercells © 1993 American Geophysical Union -- From: Church et al., The Tornado

36. 35 Classic Supercell Hook Heaviest Precipitation (core)

37. 36 Hybrids n Class distinctions are much less obvious in the real world! n Visibly a storm may look different on radar than it does in person -- makes storms difficult to classify n Supercells often evolve from LP  Classic  HP. There is a continuous spectrum of storm types.

38. 37 Supercell Evolution n Early Phase – –Initial cell development is essentially identical to that of a short-lived single cell storm. – –Radar reflectivity is vertically stacked – –Motion of the storm is generally in the direction of the mean wind – –Storm shape is circular (from above) and symmetrical

39. 38 Supercell Evolution -- Early Phase Top ViewSide View Heaviest Precipitation © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

40. 39 Supercell Evolution n Middle Phase – –As the storm develops, the strong wind shear alters the storm characteristics from that of a single cell – –The reflectivity pattern is elongated down wind -- the stronger winds aloft blow the precipitation – –The strongest reflectivity gradient is usually along the SW corner of the storm – –Instead of being vertical, the updraft and downdraft become separated

41. 40 Supercell Evolution n Middle Phase – –After about an hour, the radar pattern indicates a “weak echo region” (WER) – –This tells us that the updraft is strong and scours out precipitation from the updraft – –Precipitation aloft “overhangs” a rain free region at the bottom of the storm. – –The storm starts to turn to the right of the mean wind into the supply of warm, moist air

42. 41 Supercell Evolution -- Middle Phase Top ViewSide View Heaviest Precipitation © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

43. 42 Supercell Evolution n Mature Phase – –After about 90 minutes, the storm has reached a quasi-steady mature phase – –Rotation is now evident and a mesocyclone (the rotating updraft) has started – –This rotation (usually CCW) creates a hook-like appendage on the southwest flank of the storm

44. 43 Supercell Evolution -- Mature Phase Top ViewSide View Heaviest Precipitation © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems Hook

45. 44 Supercell Evolution -- Mature Phase Hook Echo

46. 45 Supercell Evolution n Mature Phase – –The updraft increases in strength and more precipitation, including hail, is held aloft and scoured out of the updraft – –As the storm produces more precipitation, the weak echo region, at some midlevels, becomes “bounded” – –This bounded weak echo region (BWER), or “vault,” resembles (on radar) a hole of no precipitation surrounded by a ring of precipitation

47. 46 Supercell Evolution -- Mature Phase Slice 4 km Bounded Weak Echo Region © 1990 *Aster Press -- From: Cotton, Storms

48. 47 Splitting Storms n If the shear is favorable (often a straight line hodograph), both circulations may continue to exist. n In this case the storm will split into two new storms. n If the hodograph is curved CW, the southern storm is favored. n If the hodograph is curved CCW, the northern storm is favored.

49. 48 Splitting Storms © 1990 *Aster Press -- From: Cotton, Storms

50. 49 Splitting Storms Split Left Mover Right Mover © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

51. 50 Updraft n The updraft is the rising column of air in the supercell n They are generally located on the front or right side of the storm n Entrainment is small in the core of the updraft n Updraft speeds may reach 50 m s -1 !!! n Radar indicates that the strongest updrafts occur in the middle and upper parts of the storm

52. 51 Updraft n Factors affecting the updraft speed – –Vertical pressure gradients » »Small effect but locally important » »Regions of local convergence can result in local areas of increased pressure gradients – –Turbulence – –Buoyancy » »The more unstable the air, the larger the buoyancy of the parcel as they rise in the atmosphere » »The larger the temperature difference between the parcel and the environment, the greater the buoyancy and the faster the updraft

53. 52 Structure of a Supercell Storm Meso- Cyclone

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55. 54 The Wall Cloud Meso- Cyclone

56. 55 The Wall Cloud Meso- Cyclone

57. 56 The Wall Cloud

58. 57 The Wall Cloud

59. 58 The Wall Cloud

60. 59 Supercell Downdrafts n The same forces that affect updrafts also help to initiate, maintain, or dissipate downdrafts: – –Vertical PGF – –Buoyancy (including precipitation loading) – –Turbulence n Downdraft wind speeds may exceed 40 m s -1

61. 60 Supercell Downdrafts n We shall examine two distinct downdrafts associated with supercell thunderstorms: – –Forward Flank Downdraft (FFD) – –Rear Flank Downdraft (RFD)

62. 61 Forward Flank Downdraft n Associated with the heavy precipitation core of supercells. n Air in the downdraft originates within the column of precipitation as well as below the cloud base where evaporational cooling is important. n Forms in the forward flank (with respect to storm motion) of the storm. n FFD air spreads out when it hits the ground and forms a gust front.

63. 62 Rear Flank Downdraft n Forms at the rear, or upshear, side of the storm. n Result of the storm “blocking” the flow of ambient air. n Maintained and enhanced by the evaporation of anvil precipitation. n Enhanced by mid-level dry air entrainment and associated evaporational cooling. n Located adjacent to the updraft.

64. 63 Supercell Downdrafts Inflow Forward Flank Downdraft Rear Flank Downdraft © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

65. 64 Rear Flank Downdraft n Forms at the rear, or upshear, side of the storm. n Result of the storm “blocking” the flow of ambient air. n Maintained and enhanced by the evaporation of anvil precipitation. n Enhanced by mid-level dry air entrainment and associated evaporational cooling. n Located adjacent to the updraft.

66. 65 Supercell Downdrafts Inflow Forward Flank Downdraft Rear Flank Downdraft © 1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology -- Volume II: Observations and Theory of Weather Systems

67. 66 Formation of the RFD n Imagine a river flowing straight in a smooth channel. n The water down the center flows smoothly at essentially a constant speed. n The pressure down the center of the channel is constant along the channel.

68. 67 Formation of the RFD n Let us now place a large rock in the center of the channel. n The water must flow around the rock. n A region of high pressure forms at the front edge of the rock -- Here the water moves slowly -- Stagnation Point

69. 68 Formation of the RFD n This happens in the atmosphere also! n The updraft acts a an obstruction to the upper level flow.

70. 69 Formation of the RFD n The RFD descends, with the help of evaporatively cooled air, to the ground. n When it hits the ground, it forms a gust front. Updraft FFD RFD Inflow Mid-level Flow Upper-level Flow Gust Front