severe convection and mesoscale convective systems r. a. houze lecture, indian institute of tropical...
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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
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