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