Severe Convection and Mesoscale Convective Systems R. A. Houze Lecture, Summer School on Severe and Convective Weather, Nanjing, 11 July 2011

Convective Clouds Lecture Sequence 1.Basic convective cloud types 2.Severe convection & mesoscale systems 3.Tropical cloud population 4.Convective feedbacks to large-scales 5.Extreme convection 6.Diurnal variability 7.Clouds in tropical cyclones

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

Why are there two types of cumulonimbus? What determines p ’ ?

Recall pressure perturbation is determined by

In single-cell and multi-cell thunderstorms negligible

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

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

Nonlinear processes required to form the mesocyclone Based on Rotunno 1981

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

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

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

Unidirectional shear When the hodograph is “unidirectional” PG force 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. Right mover Left mover Klemp 1987

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

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

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

Tornadogenesis

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

Factors contributing to tornado formation HELICITY MESOCYCLONE HORIZONTAL VORTICITY GENERATION

Mesoscale Convective System ~500 km

Three MCSs Mesoscale Convective System

1458GMT 13 May 2004 Convective Precipitation Stratiform Precipitation Radar Echoes in the 3 MCSs

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

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

Layer Lifting

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

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

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

100 km Vigorous convection Old convection Subdivision of precipitation of MCS into convective and stratiform components Houze 1997

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

Old Convection Heigh t Distance (V T ) snow ~1-2 m/s Houze 1997 Ice particles drift downward Melting produces “bright band”

How convective cells distribute precipitation particles in the MCS “Particle fountains” “Particle fountains” Height Yuter & Houze 1995

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

Houze 1982 Heating & Cooling Processes in an MCS Cpndensation and Deposition Melting Evaporation Convective precipitation Stratiform precipitation 30 km125 km SW LW This vertical distribution of diabatic processes applies whether the MCS is propagating or not Cloud

Simplified MCS Heating Profiles Height (km) Deg K/day Convective Stratiform Schumacher et al. 2004

✔ Conclusion of Lectures 1 & 2: We have looked at all but the TCs Stratus Stratocumulus Cumulus Cumulonimbus Mesoscale Convective System Tropical Cyclone Later

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

Convective Clouds Lecture Sequence 1.Basic convective cloud types 2.Severe convection & mesoscale systems 3.Tropical cloud population 4.Convective feedbacks to large-scales 5.Extreme convection 6.Diurnal variability 7.Clouds in tropical cyclones Next

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This research was supported by NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G, NSF grants, ATM , ATM , DOE grant DE-SC / ER-6

Columns Needles Dendrites   ColumnsPlates & Dendrites Aggregates & Drops Flight Level Temperature (deg C) Relative Frequency of Occurrence Melting Precipitation-sized Ice Particles in MCSs over the Bay of Bengal in MONEX Houze & Churchill 1987

Development of stratiform precipitation in a mesoscale convective system

Supercell Storm RainHail