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 Houze 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 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 flights Show deep layer of inflow to updrafts Kingsmill & Houze 1999

Bryan and Fritsch 2000 Analysis and simulation of midlatitude continental convection “Slab” or Layer Overturning

Height (km) Mechem et al Simulation of tropical oceanic convection

Pandya & Durran 1996 Horizontal wind Mean heating in convective line

Lower troposphere above boundary layer cooler, 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 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 Sizes of MCSs observed in TOGA COARE

Courtesy Brian Mapes Divergence Profiles of MCSs over West Pacific

Fritsch et al (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 fluxes warm low levels

Thorncroft figures From AMMA Science Plan Thorncroft et al 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”

SWNE Houze et al Stratiform region momentum transport in TOGA COARE MCS of 11 February 1993 As seen by ship radar stratiform echo Downward momentum transport “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 km

strong westerly regionwesterly onset region TOGA COARE: Ship and aircraft radar data relative to Kelvin-Rossby wave structure Houze et al Low-level flow

m/s Mechem et al Mesoscale model simulation of MCS in westerly onset regime Perturbation momentum structure

Mechem et al Mesoscale model simulation of MCS in strong westerly regime Perturbation momentum structure

Mechem et al feedback - feedback Momentum fluxes and flux convergences for simulated cases Westerly Onset Case Strong Westerly Case

Global satellite observations Global variability of MCS structure

TRMM Precipitation Radar Schumacher & Houze 2003

Hartmann et al Schumacher et al 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 85 GHz 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