Mesoscale Convective Systems Robert Houze Department of Atmospheric Sciences University of...
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Mesoscale Convective Systems
Robert HouzeDepartment of Atmospheric Sciences
University of Washington
Nebraska
Kansas
Oklahoma
Arkansas
Early View of a Mesoscale Convective System, ca 1974
Figure CONVSF
Houze 1997
100 km
Houze 1997
Precipitation in a Mesoscale Convective System
Houze 1982
Heating & Cooling Processes in an MCS
Houze 1982
Idealized Heating Profiles of MCSs
Non-dimensional Heating
Houze et al. 1989
Circulation Pattern of an MCS, ca 1989
Mesoscale circulation features identified, but suggests air enters updraft from thin surface layer
Layer lifting
TOGA COARE Airborne Doppler Observations of MCSs
25 convective region flightsShow deep layer of inflow to updrafts
Kingsmill & Houze 1999
0ze
Bryan and Fritsch 2000Analysis and simulation of midlatitude continental convection
“Slab” or Layer Overturning
Hei
gh
t (k
m)
Mechem et al. 2000Simulation of tropical oceanic convection
Pandya & Durran 1996
Horizontal wind
Mean heating in convective line
Lower troposphere above boundary layercooler, more moist, and less stable
Simulation of an MCS over the tropical ocean, near Kwajalein
Courtesy Professor Rob Fovell
Gentle, persistent lifting ahead of line
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
Houze 1982
Heating & Cooling Processes in an MCS
Figure CONVSF
Houze 1997
100 km
Houze 1997
Midlevel inflow can come from any direction
“rear inflow”
TOGA COARE Airborne Doppler Observations of MCSs
25 Stratiform region flights
Kingsmill & Houze 1999
Heating, PV generation, & upscale feedbacks
Chen et al. 1996
Sizes of MCSs observed in TOGA COARE
Courtesy Brian Mapes
Divergence Profiles of MCSs over West Pacific
Fritsch et al. 1994(based on Raymond & Jiang 1990)
PV Generation by an MCS
Chen & Frank 1993
Vortex Spinup by an MCS
Bister and Emanuel 1997
Development of a Tropical Cyclone from an MCS
Houze 1982
Idealized Heating Profiles of MCSs
Non-dimensional Heating
Stratiform region vortex builds down and sfc fluxeswarm low levels
Thorncroft figures
AEWs
MCSs
SAL
TC
AEWs
MCSs
SAL
TC
From AMMA Science PlanThorncroft et al. 2004
Interaction of MCSs with Synoptic-scale Easterly Wave
What about momentum feedbacks?
Yang & Houze 1996
Perturbation pressure field in a simulated MCS
“midlevel inflow”
Yang & Houze 1996
Momentum changes produced by different parts of simulated MCS
“midlevel inflow”
SW NE
Houze et al. 2000
Stratiform region momentum transport in TOGA COARE MCS of 11 February 1993
As seen by ship radar
stratiformecho
Downward momentumtransport
“midlevel inflow”
reflectivity
Doppler velocity
Stratiform region momentum transport in TOGA COARE MCS of 15 December 1992
As seen by ship radar
Houze et al. 2000
0.5 km
strong westerly region westerlyonset region
TOGA COARE: Ship and aircraft radar data relative to Kelvin-Rossby wave structure
Houze et al. 2000
Low-level flow
m/s
Mechem et al. 2004
Mesoscale model simulation of MCS in westerly onset regime
Perturbation momentum structure
Mechem et al. 2004
(b) 3 hu '
Mesoscale model simulation of MCS in strong westerly regime
Perturbation momentum structure
Mechem et al. 2004
+ feedback
- feedback
Momentum fluxes and flux convergences for simulated cases
Westerly OnsetCase
Strong Westerly
Case
Global satellite observations
Global variability of MCS structure
TRMM Precipitation RadarSchumacher & Houze 2003
Hartmann et al. 1984Schumacher et al. 2004
Large-scale response to precipitation heating
Most realistic when horizontal distribution of vertical profile of heating is correct
200 mb stream function
400 mb heating
4 month El Nino season 1998
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 Stratiform Rain Fraction
MCSs with large 85GHz ice scattering
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
LeMone 1983
Buoyancy Produced Pressure Minimum in an MCS