Severe Convection and Mesoscale Convective Systems

R. A. Houze

Lecture, Indian Institute of Tropical Meteorology, Pune, 5 August 2010

Clouds in Low Latitudes

Lecture Sequence1. Basic tropical cloud types

2. Severe convection & mesoscale systems

3. Tropical cloud population

4. Convective feedbacks to large-scales

5. Monsoon convection

6. Diurnal variability

7. Clouds in tropical cyclones

Continued

Two Types of Cumulonimbus

“Multicell Thunderstorm”

“Supercell Thunderstorm”

Severe Convective Storm

RainHail

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

∇2 pD

* =FD =−∇⋅ ρov⋅∇v( )

The effect of directional shear can be seen by linearizing

About a mean velocity of

v = u,v,0( )

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.

Rig

ht

mo

ver

Left mover

Klemp 1987

Clockwise hodograph

When the hodograph is “clockwise”

V P GVertical 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

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

ConvectivePrecipitation

StratiformPrecipitation

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 1996

0

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

Vigorousconvection

Oldconvection

Subdivision of precipitation of MCSinto convective and stratiform components

Houze 1997

Hei

gh

t

Distance

Vigorous Convection

Max w > (VT)snow

Houze 1997

Big particles fall out near updraft

Get vertical cores of max reflectivity

Old Convection

Hei

gh

t

Distance

(VT)snow~1-2 m/s

Houze 1997

Ice particles drift downward

Melting produces “bright band”

Columns

Needles

Dendrites

Columns Plates & Dendrites

Aggregates &Drops

Flig

ht

Lev

el T

emp

erat

ure

(d

eg C

)

0

-5

-10

-15

-20

-25

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

How convective cells distribute precipitation particles in the MCS

“Particlefountains”“Particlefountains”

Generalized structure of an MCS in shear

Houze et al. 1989

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

30 km125 km

SW

LW

LW

This vertical distribution of

diabatic processes applies whether

the MCS is propagating or not

This vertical distribution of

diabatic processes applies whether

the MCS is propagating or not

Cloud

✔

Conclusion of Lectures 1 & 2:We have looked at all but the TCs

Stratus

Stratocumulus

Cumulus

Cumulonimbus

MesoscaleConvective

System

Tropical Cyclone

Later

Summary of key pointsStratocumulus•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

Clouds in Low Latitudes

Lecture Sequence1. Basic tropical cloud types

2. Severe convection & mesoscale systems

3. Tropical cloud population

4. Convective feedbacks to large-scales

5. Monsoon convection

6. Diurnal variability

7. Clouds in tropical cyclones

Next

End

This research was supported by NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G,

NSF grants, ATM-0743180, ATM-0820586, DOE grant DE-SC0001164 / ER-6