1. Some heavy precipitation issues

2. Heavy precipitation at a location = intensity x longevity

3. Common sources of heavy precipitation in U.S. Mesoscale convective systems and vortices Orographically induced, trapped or influenced storms Landfalling tropical cyclones

4. Mesoscale Convective Systems (MCSs)

5. MCSs & precipitation facts Common types: squall-lines and supercells Large % of warm season rainfall in U.S. and flash floods (Maddox et al. 1979; Doswell et al. 1996) Initiation & motion often not well forecasted by operational models (Davis et al. 2003; Bukovsky et al. 2006) –Boundary layer, surface and convective schemes “Achilles’ heels” of regional-scale models –Improved convective parameterizations help simulating accurate propagation (Anderson et al. 2007; Bukovsky et al. 2006) Supercells often produce intense but not heavy rainfall –Form in highly sheared environments –Tend to move quickly, not stay in one place

6. U.S. flash flood seasonality Maddox et al. (1979) Contribution of warm season MCSs clearly seen Number of events

7. Linear MCS archetypes (e.g., squall-lines) Parker and Johnson (2000) 58% 19%

8. Squall-lines usually multicellular

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

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

11. Storm motion matters Doswell et al. (1996) How a storm moves over a specific location determines rainfall received

12. Storm motion matters Doswell et al. (1996)

13. Forecasting MCS motion (or lack of motion…)

14. 19980714 - North Plains http://locust.mmm.ucar.edu/episodes

15. 19980714 - North Plains

16. “Rules of thumb” “Why?” “Right for the right reason?”

17. Some common “rules of thumb” ingredients CAPE (Convective Available Potential Energy) CIN (Convective Inhibition) Precipitable water Vertical shear - magnitude and direction Low-level jet Midlevel cyclonic circulations

18. Some common “rules of thumb” MCSs tend to propagate towards the most unstable air 1000-500 mb layer mean RH ≥ 70% MCSs tend to propagate parallel to 1000-500 mb thickness contours MCSs favored where thickness contours diverge MCSs “back-build” towards higher CIN Development favored downshear of midlevel cyclonic circulations

19. Junker et al. (1999) # = precip. category 70% layer RH RH > 70% 70% RH rule of thumb Implication: Relative humidity more skillful than absolute humidity

20. MCSs tend to follow thickness contours Implication: vertical shear determines MCS orientation and motion. Thickness divergence likely implies rising motion

21. Back-building towards higher CIN Lifting takes longer where there is more resistance

22. Corfidi vector method Cell motion vs. system motion

23. Corfidi vector method Propagation is vector difference P = S - C Therefore, S = C + P to propagate = to cause to continue, to pass through (space)

24. Example

25. Schematic example A multicellular squall-line

26. Schematic example Cell motion as shown

27. Schematic example System motion as shown

28. Schematic example We wish to forecast system motion So we need to understand what controls cell motion and propagation

29. Individual cell motion Cells tend to move at 850-300 mb layer wind speed* Corfidi et al. (1996) “Go with the flow” Agrees with previous observations (e.g, Fankhauser 1964) and theory (classic studies of Kuo and Asai) *Layer wind weighted towards lower troposphere, using winds determined around MCS genesis. Later some slight deviation to the right often appears

30. Individual cell motion Cells tend to move at 850-300 mb layer wind speed Cell direction comparable To 850-300 mb layer wind direction Corfidi et al. (1996)

31. Composite severe MCS hodograph Bluestein and Jain (1985) Selective composite already excludes non-severe, non-TS squalls

32. Composite severe MCS hodograph Bluestein and Jain (1985)

33. Composite severe MCS hodograph Bluestein and Jain (1985)

34. Composite severe MCS hodograph Low-level jets (LLJs) are common Note P ~ -LLJ Bluestein and Jain (1985)

35. Propagation vector and LLJ Many storm environments have a low-level jet (LLJ) or wind maximum Propagation vector often anti-parallel to LLJ Corfidi et al. (1996) Propagation vector direction P ~ -LLJ

36. Forecasting system motion using antecedent information Cell motion ~ 850-300 mb wind Propagation ~ equal/opposite to LLJ S = C - LLJ

37. Evaluation of Corfidi method Method skillful in predicting system speed and direction Corfidi et al. (1996)

38. Limitations to Corfidi method Wind estimates need frequent updating Influence of topography on storm initiation, motion ignored Some storms deviate significantly from predicted direction (e.g., bow echoes) P ~ -LLJ does not directly capture reason systems organize (shear) or move (cold pools) Beware of boundaries! Corfidi (2003) modified vector method

39. Composite severe MCS hodograph Low-level shear influences storm organization & motion Angle between lower & upper shear also important (Robe and Emanuel 2001) Bluestein and Jain (1985)

40. http://locust.mmm.ucar.edu/episodes Low-level shear

41. 5 June 2004 X = Hays, Kansas, USA

42. 5 June 2004 X = Hays, Kansas, USA

43. Mesoscale Convective Vortices (MCVs)

44. Potential vorticity Simplest form (see Holton. Ch. 4): absolute vorticity/depth is conserved for dry adiabatic processes. Equivalent to angular momentum conservation; stretching increases vorticity. This is a special case of Rossby-Ertel PV

45. Rossby-Ertel potential vorticity q incorporating: 3D vorticity vector, potential temperature gradient and Coriolis expressed as a vector (function of z only) In this formulation, mass x q is conserved between two isentropes even (especially!) if diabatic processes are changing the potential temperature Haynes and McIntyre (1987)

46. Rossby-Ertel potential vorticity q Here, we simplify a little bit and focus only on the vertical direction. The conserved quantity is mq. Holton’s version is derivable from Rossby-Ertel’s equation, where A is horizontal area. (Keep in mind ∆  is fixed between two isentropes.)

47. Rossby-Ertel PV For a dry adiabatic process, the mass between two isentropes cannot change. Thus, the only way to increase the cyclonic vorticity  is to move the object equatorward (decreasing f) OR decrease its horizontal area A. Now, consider a more relevant example…

48. Start with a stably stratified environment, with no initial horizontal variation. Define two layers, bounded by these three isentropes. We are dealing with horizontal layers. Horizontal area A is not relevant.

49. m 1 and m 2 are the initial masses residing in these two layers. q 1 and q 2 are the initial PVs. mq can be transported horizontally but not vertically. So m 1 q 1 and m 2 q 2 will not change.

50. Introduce a diabatic heat source, representing convection. The potential temperature in the heated region increases. This effectively moves the isentrope  2 downward.

51. Now there is less mass in the lower isentropic layer, and more mass in the upper layer. Because mq is conserved between any two isentropes, q has increased in the lower layer because m has decreased there. q has NOT been advected vertically.

52. The increased q in the lower layer represents a positive PV anomaly (+PV). Because q has increased,  is enhanced and a cyclonic circulation is induced. In the upper layer, decreased q means -PV and an induced anticyclonic circulation.

53. Cyclonic vortex following squall line Not a clean MCV case

54. MCVs as PV anomalies Raymond and Jiang (1990) MCV is a midlevel positive PV anomaly that can be created by a squall-line. We are ignoring the negative PV anomaly farther aloft.

55. MCVs as PV anomalies PV anomaly shown drifting in westerly sheared flow Raymond and Jiang (1990)

56. MCVs as PV anomalies Adiabatic ascent induced beneath anomaly Raymond and Jiang (1990)

57. MCVs as PV anomalies Ascent occurs on windward (here, east) side… destabilization Raymond and Jiang (1990)

58. MCVs as PV anomalies Westerly vertical shear implies isentropes tilt upwards towards north Raymond and Jiang (1990)

59. MCVs as PV anomalies Cyclonic circulation itself results in ascent on east side Raymond and Jiang (1990)

60. MCVs as PV anomalies Combination: uplift & destabilization on windward side AND downshear side Raymond and Jiang (1990)

61. Composite analysis of MCV heavy rain events Schumacher and Johnson (2008) 600 mb vorticity (color), heights and winds. Map for scale only Based on 6 cases poorly forecasted by models Composite at time of heaviest rain (t = 0h) Heaviest rain in early morning Heaviest rain south of MCV in 600 mb trough

62. Schumacher’s situation Midlevel MCV See also Fritsch et al. (1994)

63. Schumacher’s situation Nocturnally-enhanced LLJ transports high  e air; Flow below PV anomaly from SW

64. Schumacher’s situation “Hairpin” hodograph: Sharp flow reversal above LLJ

65. Schumacher’s situation South side of MCV is windward at low-levels and downshear relative to midlevel vortex

66. Schumacher’s situation Tends to result in very slow-moving, back-building convection south of MCV

67. Back-building Schumacher and Johnson (2005) Doswell et al. (1996) Ground-relative system speed ~ 0

68. Evolution of the heavy rain event 600 mb vorticity, 900 mb winds & isotachs At t - 12h (afternoon): - MCV located farther west - 900 mb winds fairly light Schumacher and Johnson (2008)

69. Evolution of the heavy rain event 600 mb vorticity, 900 mb winds & isotachs At t - 6h (evening): - MCV drifted west - 900 mb winds strengthening (LLJ intensifying) Schumacher and Johnson (2008)

70. Evolution of the heavy rain event 600 mb vorticity, 900 mb winds & isotachs At time of heaviest rain (midnight): - 900 mb jet well developed - LLJ located east, south of MCV Schumacher and Johnson (2008)

71. Evolution of the heavy rain event 600 mb vorticity, 900 mb winds & isotachs At t + 6h (morning): rain decreases as LLJ weakens Schumacher and Johnson (2008)

72. The South Plains nocturnal low-level jet (LLJ)

73. South Plains LLJ Enhanced southerly flow over South Plains Most pronounced at night Responsible for moisture advection from Gulf & likely a major player in nocturnal thunderstorms and severe weather

74. Bonner (1968) - LLJ occurences meeting certain criteria - most frequent in Oklahoma - most frequent at night

75. Explanations for LLJ Oscillation of boundary layer friction (mixing) responding to diurnal heating variation Vertical shear responding to diurnally varying west-east temperature gradients owing to sloped topography Cold air drainage down the Rockies at night Topographic blocking of some form

76. Bonner (1968) observations of wind speed vs. height for days in which nocturnal LLJ appeared at Ft. Worth, TX -wind speed max just below 1 km MSL (about 800 m AGL) at midnight and 6AM local time - note increased low-level shear

77. Bonner (1968) observations of Ft. Worth wind at height of wind max. wind weaker, more southerly during afternoon nighttime wind stronger, more from southwest, elevation lower

78. Episodes of MCSs & predictability Carbone et al. (2002) Hovmoller diagrams reveal westward- propagating MCSs Note “envelope” of several systems with “connections”

79. MCV role in predictability Carbone et al. (2002)

80. “Training lines” of cells Schumacher and Johnson (2005) In Asia, stationary front could be the Mei-Yu (China), Baiu (Japan) or Changma (Korea) front Motion along the front and/or continuous back- building

81. Sun and Lee (2002) Record 619 mm in 15 h at Ganghwa, Korea X Lee et al. (2008) shear

82. 2-3 April 2006

84. Why did new cells appear ahead of the mature line? Effectively “speeds up” (earlier rain) & “slows down” (prolongs rain)”

85. New cell initiation ahead of squall-lines Fovell et al. (2006) Unsteady multicellularity excites internal gravity waves

86. New cell initiation ahead of squall-lines One possible trapping mechanism: the storm anvil Fovell et al. (2006)

87. Trapping mechanism Trapping can occur when a layer of lower l 2 resides over a layer with higher values More general Scorer parameter (c = wave speed) Lowered l2 can result from decreased stability or creation of a jet-like wind profile –Storm anvil does both

88. New cell initiation ahead of squall-lines The waves themselves disturb the storm inflow Fovell et al. (2006)

89. New cell initiation ahead of squall-lines …and can create clouds Fovell et al. (2006)

90. New cell initiation ahead of squall-lines …some of which can develop into precipitating, even deep, convection Fovell et al. (2006)

91. New cell initiation ahead of squall-lines Other plausible mechanisms for new cell initiation exist Fovell et al. (2006)

92. New cell initiation ahead of squall-lines 150 km 14 km Fovell et al. (2006)

93. New cell initiation ahead of squall-lines 150 km 14 km Fovell et al. (2006)

94. New cell initiation ahead of squall-lines 150 km 14 km Fovell et al. (2006)

95. Importance of antecedent soil moisture conditions (Generally not captured well by models)

96. Tropical Storm Erin (2007) http://en.wikipedia.org/wiki/Image:Erin_2007_track.png

97. TS Erin well inland Kevin Kloesel, University of Oklahoma

98. Erin’s redevelopment over Oklahoma Emanuel (2008) http://www.meteo.mcgill.ca/cyclone/lib/exe/fetch.php?id=start&cache=cache&media=wed2030.ppt

99. Erin inland reintensification Hot and wet loamy soil can rapidly transfer energy to atmosphere Previous rainfall events left Oklahoma’s soil very wet Need to consider antecedent soil moisture and soil type Emanuel (2008) see also Emanuel et al. (2008)

100. Soil T as Erin passed Emanuel (2008)

101. Summary A critical view of some ideas, tools relevant to heavy precipitation forecasting Emphasis on factors operational models do not handle particularly well –CAPE & CIN, MCS development and motion, surface and boundary layer conditions

102. end

103. Composite sounding (at heavy rain location) Southwesterly winds beneath MCV Westerly winds at midlevels push MCV eastward Southerly shear at midlevels across MCV Schumacher and Johnson (2008)

104. MCV-shear interaction decreases CIN CIN (colored) and CAPE (contours) t - 6h: Moderate CAPE (1500 J/kg) CIN 10-50 J/kg Schumacher and Johnson (2008)

105. MCV-shear interaction decreases CIN CIN (colored) and CAPE (contours) t - 6h: Moderate CAPE (1500 J/kg) CIN 10-50 J/kg t - 0h: CAPE unchanged CIN disappeared owing to MCV uplift Schumacher and Johnson (2008)

106. Linear MCS archetypes Parker and Johnson (2000) TS - Trailing Stratiform 58% LS - Leading Stratiform 19% PS - Parallel Stratiform 19%

107. http://locust.mmm.ucar.edu/episodes