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.
2.
3.
4.
5.
6.
7.
Basic convective cloud types
Severe convection & mesoscale systems
Tropical cloud population
Convective feedbacks to large-scales
Extreme convection
Diurnal variability
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
Tilting of the environment shear & “storm splitting”
Assume
unidirectional
shear
min p’
tilting of environment
vorticity  vortex  min
p’
PG force Storm “splits” as a result
of this rotationdetermined vertical force
End up with two storms!
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
(
Ñ2 p*D = FD = -Ñ × ro v × Ñv
About a mean velocity of
(
v = u,v ,0
)
Which leads to
Where S is the environment shear
)
Middle level of storm
S
This implies lifting at
low levels on
downshear side of
storm.
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
Klemp 1987
When the hodograph is “clockwise”
Vertical
pressure
VPG
gradient forces
updraft on the
right flank;
downdraft on
left flank.
Clockwise
hodograph
Right-mover
favored
Klemp 1987
Probable Location of Tornadic Thunderstorms
Tornado environment
sounding
CU
PU “cap”
T
CU
Tornado (T)
forms where
wind pattern
creates strong
combination of
CU and PU
Probable Location of Tornadic Thunderstorms
Tornado environment
hodograph
Note some shear
is in the
boundary layer
T
Tornado (T)
forms where
the shear is
both strong &
directional
Tornadogenesis
Further considerations for tornadic
storms:
•Shear in boundary layer (“helicity”)
•Generation of vorticity by the storm
Factors contributing to tornado formation
MESOCYCLONE
HELICITY
HORIZONTAL
VORTICITY
GENERATION
Mesoscale Convective System
~500 km
Mesoscale Convective System
Three MCSs
Radar Echoes in the 3 MCSs
1458GMT 13 May 2004
Stratiform
Precipitation
Convective
Precipitation
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
Mean heating in convective line
Horizontal wind
Mesoscale response to
the heating in the line
0
Pandya & Durran 1996
Vorticity interpretation
When an MCS forms in a sheared environment, solutions to 2D
vorticity equation look like this:
B>0
Shear
Moncrieff 1992
Vorticity interpretation
Model results are consistent with the theory
Horizontal
vorticity
generated by
the line of
convection
B>0
Get updraft in
the form of a
deep layer of
ascending
front-to-rear
flow
Fovell & Ogura 1988
Subdivision of precipitation of MCS
into convective and stratiform components
Old
convection
Vigorous
convection
100 km
Houze 1997
Vigorous Convection
Max w > (VT)snow
Height
Big particles fall
out near updraft
Get vertical cores
of max reflectivity
Distance
Houze 1997
Old Convection
Heigh
t
(VT)snow~1-2 m/s
Ice particles drift
downward
Melting produces
“bright band”
Distance
Houze 1997
Height
How convective cells distribute precipitation
particles in the MCS
“Particle
fountains”
Yuter & Houze 1995
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
Heating & Cooling Processes in an MCS
SW
Cloud
Cpndensation
and
Deposition
LW
This vertical
distribution of
diabatic processes
applies whether
the MCS is
propagating or not
Melting
LW
Evaporation
125 km
30 km
Stratiform
Convective
precipitation precipitation
Houze 1982
Simplified MCS Heating Profiles
Height (km)
Stratiform
Convective
Schumacher et al. 2004
Deg K/day
Conclusion of Lectures 1 & 2:
We have looked at all but the TCs
Cumulonimbus
Cumulus
Mesoscale
Convective
System
Stratocumulus
Later
✔
Stratus
Tropical Cyclone
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.
2.
3.
4.
5.
6.
7.
Basic convective cloud types
Severe convection & mesoscale systems
Next
Tropical cloud population
Convective feedbacks to large-scales
Extreme convection
Diurnal variability
Clouds in tropical cyclones
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
Flight Level Temperature (deg C)
Precipitation-sized Ice Particles in MCSs
over the Bay of Bengal in MONEX
-25
-20
Columns
Columns
Plates &
Dendrites
Aggregates &
Drops
Dendrites
-15
-10
-5
0
Needles
*
*
Melting
Relative Frequency of Occurrence
Houze & Churchill 1987
Development of stratiform precipitation in a
mesoscale convective system
Supercell Storm
Hail
Rain