Mesoscale Convective Systems
Robert Houze
Department of Atmospheric Sciences
University of Washington
Nebraska
Kansas
Oklahoma
Arkansas
Early View of a Mesoscale
Convective System, ca 1974
Figure CONVSF
Precipitation in a Mesoscale
Convective System
Houze 1997
100 km
Houze 1997
Heating & Cooling Processes in an MCS
Houze 1982
Idealized Heating Profiles of MCSs
Non-dimensional Heating
Houze 1982
Circulation Pattern of an MCS, ca 1989
Mesoscale circulation features identified, but suggests air enters updraft from thin
surface layer
Houze et al. 1989
Layer lifting
TOGA COARE
Airborne Doppler Observations of MCSs
25 convective region flights
Show deep layer of inflow to updrafts
 e
0
z
Kingsmill & Houze 1999
Bryan and Fritsch 2000
Analysis and simulation of midlatitude continental convection
“Slab” or Layer Overturning
Height (km)
Mechem et al. 2000
Simulation of tropical oceanic convection
Pandya & Durran 1996
Mean heating in convective line
Horizontal wind
Simulation of an MCS over the tropical ocean, near Kwajalein
Courtesy Professor Rob Fovell
Gentle, persistent lifting ahead of line
Lower troposphere above boundary layer
cooler, more moist, and less stable
Discrete Propagation
Loop showing tropical discrete propagation in an MCS
over Oklahoma
Courtesy Professor Rob Fovell
Loop showing tropical discrete propagation in an MCS
over the Bay of Bengal
Midlevel Inflow
Heating & Cooling Processes in an MCS
Houze 1982
Figure CONVSF
Midlevel inflow can come from any direction
Houze 1997
100 km
Houze 1997
TOGA COARE
Airborne Doppler Observations of MCSs
25 Stratiform region flights
Kingsmill & Houze 1999
Heating, PV generation, & upscale feedbacks
Sizes of MCSs observed in TOGA COARE
Chen et al. 1996
Divergence Profiles of MCSs over West Pacific
Courtesy Brian Mapes
PV Generation by an MCS
Fritsch et al. 1994
(based on Raymond & Jiang 1990)
Vortex Spinup by an MCS
Chen & Frank 1993
Development of a Tropical Cyclone from an MCS
Bister and Emanuel 1997
Idealized Heating Profiles of MCSs
Stratiform region vortex
builds down and sfc fluxes
warm low levels
Non-dimensional Heating
Houze 1982
Interaction of MCSs with Synoptic-scale Easterly Wave
SAL
AEWs
Thorncroft figures
TC
MCSs
From AMMA Science Plan
Thorncroft et al. 2004
What about momentum feedbacks?
Perturbation pressure field in a simulated MCS
Yang & Houze 1996
Momentum changes produced by different parts of simulated MCS
Yang & Houze 1996
Stratiform region momentum transport in TOGA COARE
MCS of 11 February 1993
As seen by ship radar
reflectivity
stratiform
echo
SW
NE
Doppler velocity
Downward
momentum
transport
Houze et al. 2000
Stratiform region momentum transport in TOGA COARE
MCS of 15 December 1992
As seen by ship radar
0.5 km
Houze et al. 2000
TOGA COARE: Ship and aircraft radar data relative to Kelvin-Rossby wave
structure
Low-level flow
strong westerly region
westerly
onset region
Houze et al. 2000
Mesoscale model simulation of MCS in westerly onset regime
Perturbation momentum structure
m/s
Mechem et al. 2004
Mesoscale model simulation of MCS in strong westerly regime
Perturbation momentum structure
(b) 3 hu '
Mechem et al. 2004
Strong
Westerly
Case
Momentum fluxes and
flux convergences for
simulated cases
+ feedback
Westerly
Onset
Case
- feedback
Mechem et al. 2004
Global satellite observations
Global variability of MCS structure
TRMM Precipitation Radar
Schumacher & Houze 2003
Large-scale response to precipitation heating
Hartmann et al. 1984
Schumacher et al. 2004
400 mb
heating
200 mb
stream
function
4 month El Nino season 1998
Most realistic when horizontal distribution of
vertical profile of heating is correct
The variation of stratiform and convective structure of MCSs is
most pronounced between land & ocean
TRMM view of Africa vis a vis the Atlantic
AMMA Science Plan, Thorncroft 2004
Rain
MCSs with large 85
GHz ice scattering
Stratiform Rain Fraction
Lightning
India: Another example of continental MCS
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
• Discrete propagation (as opposed to lifting over cold pool) is an
significant component of the system motion
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
• Discrete propagation (as opposed to lifting over cold pool) is an
significant component of the system motion
• Midlevel inflow direction controlled by large-scale environment
relative flow
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
• Discrete propagation (as opposed to lifting over cold pool) is an
significant component of the system motion
• Midlevel inflow direction controlled by large-scale environment
relative flow
• Positive PV develops in the cloud layer of the stratiform region and
can lead to tropical cyclone formation and possibly feedback upscale
to synoptic-scale waves
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
• Discrete propagation (as opposed to lifting over cold pool) is an
significant component of the system motion
• Midlevel inflow direction controlled by large-scale environment
relative flow
• Positive PV develops in the cloud layer of the stratiform region and
can lead to tropical cyclone formation and possibly feedback upscale
to synoptic-scale waves
• Momentum generation in stratiform region can be significant and
have either positive or negative upscale feedbacks to large scale flow
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
• Discrete propagation (as opposed to lifting over cold pool) is an
significant component of the system motion
• Midlevel inflow direction controlled by large-scale environment
relative flow
• Positive PV develops in the cloud layer of the stratiform region and
can lead to tropical cyclone formation and possibly feedback upscale
to synoptic-scale waves
• Momentum generation in stratiform region can be significant and
have either positive or negative upscale feedbacks to large scale flow
• Large-scale response to MCS heating depends on the global
variability of stratiform rain fraction
Summary
• MCSs have rain areas ~hundreds of kilometers in scale
• Stratiform region has cooling at low levels & warming at upper levels
• Updrafts are fed by a deep layer, which is a mesoscale response to
the net heating profile of the system
• Discrete propagation (as opposed to lifting over cold pool) is an
significant component of the system motion
• Midlevel inflow direction controlled by large-scale environment
relative flow
• Positive PV develops in the cloud layer of the stratiform region and
can lead to tropical cyclone formation and possibly feedback upscale
to synoptic-scale waves
• Momentum generation in stratiform region can be significant and
have either positive or negative upscale feedbacks to large scale flow
• Large-scale response to MCS heating depends on the global
variability of stratiform rain fraction
• Biggest differences in MCS structure are between land and ocean;
over land get lower stratiform rain fraction, more ice scattering at 85
GHz, and more lightning.
End
Buoyancy Produced Pressure Minimum in an MCS
LeMone 1983