1. Background and Definitions
Mesoscale Convective System (MCS): an isolated, nearly contiguous region of thunderstorms, sometimes surrounded by an extensive region of moderate rainfall. Total size is usually km across.
Bow-echo: a bow-shaped line of thunderstorms often containing strong surface winds.
Mesoscale Convective Vortex: a lower-mid-tropospheric horizontal wind circulation derived from an area of convection (often an MCS).
0600 UTC 10 June, 2003 X
200 km
0540 UTC 10 June, 2003
11 June, 2003

2. Houze et al. (BAMS, 1989)

3. Houze et al. (BAMS, 1989)

4. Johnson and Hamilton (1988)

5. ⁄ Basic Equations: 2D Squall Line
*Also, no vortex tilting or stretching --
Or, more simply, consider the 2D horizontal vorticity equation:
where

7. “Optimal” condition for cold pool lifting C/∆u = 1
RKW Theory
Rotunno et al. (JAS, 1988)
C/∆u > 1
“Optimal” condition for cold pool lifting
C/∆u = 1
C/∆u < 1

8. 2D Convective System Evolution:
C/∆u << 1
C/∆u ~ 1
C/∆u > 1
Weak shear, strong cold pool: rapid evolution
Strong shear, weak cold pool: slow evolution

10. Rear-inflow jet intensity increases with increasing CAPE!

12. RKW Theory: all other things being equal (e. g
RKW Theory: all other things being equal (e.g., same external forcing), squall line strength/longevity is “optimized” when the circulation associated with the system-generated cold pool remains “in balance” with the circulation associated with the low-level vertical wind shear.
Issue: Squall-lines are observed to be strong and long-lived for a wider range of environments than suggested by the models (e.g., weaker shears, deeper shears,….).
“Optimality” depends on metric being applied… e.g., updraft strength, rainrate, total rainfall, maximum surface winds....

13. Now Consider a 3D Squall Line….without Coriolis:⁄ ⁄ --

14. Weisman and Davis (1998)
f=0

15. How can we systematically produce the observed line-end vortex pattern?

16. Line-end vortex mechanisms:
Mature Phase:

17. ⁄
Vertical Vorticity: ⁄
…flux form ⁄
Circulation:

19. ⁄ ⁄

20. …tilting of system-generated horizontal vorticity
Rear-inflow jet
(Davis and Weisman, 1994; Weisman and Davis, 1998; Davis and Galarneau, 2009)

21. Role of Line-End Vortices
Focuses and Intensifies Rear-Inflow Jet

23. Now Consider a 3D Squall Line….with Coriolis:--

24.   
f-flux  ⁄
f-flux

26. Derechos: Severe Lines of Thunderstorms
Damage from straight-line wind
Long swaths (> 400 km), long duration (> 6 h)
Wide damage swaths ( km)
Rapid movement: m/s
Earthsky.org
NOAA Storm Prediction Center
Csmonitor.com

27. Derecho: (Johns and Hirt 1987)
Large CAPE
Moderate Shear

29. 29 June 2012 Derecho: Composite Radar 03 UTC 18 UTC 21 UTC 00 UTC
SPC Storm Reports

30. 29 June 2012 Derecho:
3 km WRF-ARW Forecast: DART Analysis, MYJ, Morrison
18 UTC (6 h)
21 UTC (9 h)
00 UTC (12 h)
03 UTC (15 h)
Composite Radar
18 UTC
21 UTC
00 UTC
03 UTC

31. 29 June 2012 21 UTC Reflectivity, Sfc winds *Cold Pool : -14 to -16 C
*Strong Rear Inflow Jet
*No cyclonic vorticity along leading line
975 hPa Theta
850 hPa Theta

32. 29 June 2012 21 UTC Vertical X-sections
*Cold Pool : 4 km deep, -14 to -16 C
*Deep Rear Inflow Jet
8 km
4 km
0 km
Reflectivity, Theta
Winds, Theta-E

34. The 8 May 2009 “Super Derecho”:
SUNY Albany 9 April 2014
The 8 May 2009 “Super Derecho”:
8-10 h of Hurricane-Force Winds, Extensive Damage…
Radar 17:56 UTC 05/08/09 (Paducah)
Morris Weisman NCAR/MMM
Also: Lance Bosart, Clark Evans

35. Base Reflectivity 1334z KSGF
KT winds at ~1kft
Here is the corresponding reflectivity image.
Base Reflectivity 1334z KSGF

36. Storm-Relative Velocity 1334z KSGF
A close-up view of the Doppler velocity data from Springfield, Missouri WSR-88D. Doppler wind speeds were in excess of 100kt at about 1000 feet AGL. All sorts of interesting couplets are also evident along the line of deep convection ahead of the circulation.
Storm-Relative Velocity 1334z KSGF

37. Occluding Stage: 09 UTC (21 h) 11 UTC (23 h) 13 UTC (25 h)

38. 850 mb W (contoured) and Vertical Vorticity (shaded)
06 UTC
07 UTC
09 UTC
11 UTC
12 UTC
13 UTC

39. Vorticity Equation:
Vertical Vorticity:
tilting
stretching

40. 07 UTC
850 hPa Vorticity
…Tilting…
Stretching

41. Circulation:
(other)

42. 
f-flux  
vort-flux  ⁄
+ (other)
tilting
vort-flux
f-flux

44. 900 hPa Horizontal Vorticity, SR Flow, W (shaded)
08 May 2009 Derecho
900 hPa Horizontal Vorticity, SR Flow, W (shaded)
With low-level jet from SW, streamwise horizontal vorticity evident in low-level environment….

45. So, is this a “Landicane”??

46. 29 June 2012 versus 08 May 2009 Derechoes
…Cold-pool dominant
…Descending rear-inflow
…Cyclonic mid-level
vortex
Radar Reflectivity
Model Reflectivity
08 May 2009
…Mesovortex dominant
…Elevated rear-inflow jet
…Warm-core vortex
extending to surface

47. Cape/Shear Intercomparison:
29 June 2012
08 May 2009
CAPE: j/kg
Shear: kts (15-25 ms-1)
CAPE: j/kg
Shear: kts (10-15 ms-1)

48. 850 hPa Intercomparison:   08 May 2009 29 June 2012
NO Low-Level Jet, NO west-east boundary
Low-Level Jet, west-east boundary, Lee trough

49. Summary:
….3 km WRF-ARW was capable of not only predicting the potential for two high-end Derecho events, but also was capable of distinguishing the differing mechanisms…
29 June 2012: Cold Pool dominant
08 May 2009: Mesovortex dominant
….These two cases may help clarify the differing environmental characteristics that contribute to these two archetypes:
29 June 2012: Extreme instability, modest
unidirectional low-level shear
08 May 2009: Mid-trop baroclinicity, low-level
jet, strong directional shear
(streamwise at low-levels)

50. Atkins et al. MWR (2004)

51. Atkins et al. MWR (2004)

52. Wakimoto et al. MWR (2006)

53. f=0 US = 20/2.5 t = 4 hr w, V z=3 km qr, V θ’ z=250 m 50 km no meso-
vortices!
continuous
updraft
50 km

54. US = 20/2.5 t = 4 hr w, V z=3 km qr, V θ’ z=250 m 50 km locational
bias
50 km

55. ⁄

56. Weisman and Trapp (2003)

57. Weisman and Trapp (2003)

58. Weisman and Trapp (2003)

59. Weisman and Trapp (2003)

60. Weisman and Trapp (2003)

61. Trapp and Weisman (2003)

62. Trapp and Weisman (2003)

63. Atkins and Laurent, MWR, 2009

64. Atkins and Laurent, MWR, 2009

65. Thompson et al., WAF 2012
0-1 km BWD (bulk wind difference)

67. Example of a “Serial” MCV/MCS Case
0915 UTC 27 May 1998
0015 UTC 28 May 1998
0715 UTC 28 May 1998
2315 UTC 28 May 1998
0515 UTC 29 May 1998
1215 UTC 29 May 1998

68. IOP 1
200 km

69. Widespread Instability
IOP 8
Mean Wind Profile
900 hPa
1730 UTC 11 June
dBz 70 60 50 X 40 30 20 10
Widespread Instability
m/s

70. Equivalent Potential Vorticity:
= 0 for isentropic motions
Equivalent Potential Vorticity:

71. Potential Vorticity:
= 0 for isentropic motions
Vorticity:

73. Long-time Behavior of MCSs
Cool H
Warm L H
Cool
(twice)

74. Raymond and Jiang (JAS 1990) Conceptual Model of Isentropic
Lifting within a Steady Balanced Vortex (e.g., MCV)

75. MCV Induced Lifting and Destabilization
Fritcsh et al. 1994, MWR

76. Low-Level Jet Scenario

77. Flash-Flood-Producing Convective Systems Associated with Mesoscale Convective Vortices
Russ Schumacher and Richard Johnson WAF (2008)

80. …integrate along a parcel’s path:
Vorticity Equation:
Vertical Vorticity:
…integrate along a parcel’s path:
tilting
stretching