Severe Convection and Mesoscale Convective Systems

1. Severe Convection and Mesoscale Convective Systems R. A. Houze Lecture, Indian Institute of Tropical Meteorology, Pune, 5 August 2010

2. Clouds in Low Latitudes Lecture Sequence • Basic tropical cloud types • Severe convection & mesoscale systems • Tropical cloud population • Convective feedbacks to large-scales • Monsoon convection • Diurnal variability • Clouds in tropical cyclones Continued

3. Two Types of Cumulonimbus “Multicell Thunderstorm” “Supercell Thunderstorm”

4. Hail Rain Severe Convective Storm

5. Why are there two types of cumulonimbus? What determinesp’?

6. Recall pressure perturbation is determined by

7. In single-cell and multi-cell thunderstorms negligible

8. Strong rotation in cloud produces cyclostrophic pressure minima in the cloud  dynamic forcing becomes important! This changes the storm from multicell to supercell

9. Tilting of the environment shear & “storm splitting” Assume unidirectional shear tilting of environment vorticity  vortex  min p’ min p’ PG force Storm “splits” as a result of this rotation-determined vertical force End up with two storms! Klemp 1987

10. Nonlinear processes required to form the mesocyclone Based on Rotunno1981

11. Why don’t we get two storms? Directional shear

12. The effect of directional shear can be seen by linearizing About a mean velocity of Which leads to Where S is the environment shear

13. Middle level of storm This implies lifting at low levels on downshear side of storm. S

14. Left mover When the hodograph is “unidirectional” Unidirectional shear In addition to pressure forces that cause storm splitting, vertical pressure gradient forces updraft on downshear side of storm, so storm BOTH splits AND moves forward. PG force Right mover Klemp 1987

15. Left mover When the hodograph is “clockwise” Vertical pressure gradient forces updraft on the right flank; downdraft on left flank. Clockwise hodograph V P G Right-mover favored Klemp 1987

16. T Tornado environment sounding CU PU “cap” CU Probable Location of Tornadic Thunderstorms Tornado (T) forms where wind pattern creates strong combination of CU and PU

17. T Probable Location of Tornadic Thunderstorms Tornado environment hodograph Note some shear is in the boundary layer Tornado (T) forms where the shear is both strong & directional

18. Tornadogenesis

19. Further considerations for tornadic storms: • Shear in boundary layer (“helicity”) • Generation of vorticity by the storm

20. Factors contributing to tornado formation MESOCYCLONE HORIZONTAL VORTICITY GENERATION HELICITY

21. Mesoscale Convective System ~500 km

22. Three MCSs Mesoscale Convective System

23. Radar Echoes in the 3 MCSs 1458GMT 13 May 2004 StratiformPrecipitation ConvectivePrecipitation

24. When convection organizes into a mesoscale convective system • parcel theory doesn’t apply • layer lifting occurs

25. Parcel Model of Convection Parcels of air arise from boundary layer This doesn’t apply to mature MCS

26. Layer Lifting

27. Gravity Wave Interpretation Mean heating in convective line Horizontal wind Mesoscale response to the heating in the line 0 Pandya & Durran 1996

28. Vorticity interpretation When an MCS forms in a sheared environment, solutions to 2D vorticity equation look like this: B>0 Shear Moncrieff 1992

29. Vorticity interpretation Model results are consistent with the theory B>0 Get updraft in the form of a deep layer of ascending front-to-rear flow Horizontal vorticity generated by the line of convection Fovell & Ogura 1988

30. Oldconvection Vigorousconvection 100 km Subdivision of precipitation of MCSinto convective and stratiform components Houze 1997

31. Height Distance Vigorous Convection Max w > (VT)snow Big particles fall out near updraft Get vertical cores of max reflectivity Houze 1997

32. Height Distance Old Convection (VT)snow~1-2 m/s Ice particles drift downward Melting produces “bright band” Houze 1997

33. Precipitation-sized Ice Particles in MCSs over the Bay of Bengal in MONEX -25 Columns Plates & Dendrites Aggregates &Drops Columns -20 Dendrites -15 Flight Level Temperature (deg C) -10 * * Needles -5 0 Melting Relative Frequency of Occurrence Houze & Churchill 1987

34. Development of stratiform precipitation in a mesoscale convective system

35. How convective cells distribute precipitation particles in the MCS “Particle fountains”

36. Generalized structure of an MCS in shear • This type of MCS propagates with a • leading line of convection, aided by downdraft cold pool, and • trailing stratiform precipitation Storm motion Sheared flow leads to older convective elements being advected rearward SF precipitation area is to the rear. Houze et al. 1989

37. Heating & Cooling Processes in an MCS SW Cloud Deposition LW This vertical distribution of diabatic processes applies whether the MCS is propagating or not Melting Evaporation LW 125 km 30 km Stratiform precipitation Convective precipitation Houze 1982

38. Conclusion of Lectures 1 & 2: We have looked at all but the TCs Cumulonimbus Cumulus MesoscaleConvectiveSystem Stratocumulus Later Stratus Tropical Cyclone ✔

39. Summary of key points • Stratocumulus • Turbulence • Entrainment • Radiation • Drizzle • Cumulus & Cumulonimbus • Buoyancy • Entrainment • Anvil cloud & thunderstorms • Intensity over land & ocean • Pressure perturbations • Vorticity • Intense Cumulonimbus • Rotation • Speed and directional shear • Mesoscale Convective Systems • Layer lifting • Convective vs stratiform precipitation • Heating profiles

40. Clouds in Low Latitudes Lecture Sequence • Basic tropical cloud types • Severe convection & mesoscale systems • Tropical cloud population • Convective feedbacks to large-scales • Monsoon convection • Diurnal variability • Clouds in tropical cyclones Next

41. End

42. This research was supported by NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G, NSF grants, ATM-0743180, ATM-0820586, DOE grant DE-SC0001164 / ER-6