Convective Feedbacks to Large
Scales: Physical Mechanisms
Radiation
Latent Heat
Vapor
Momentum
R. A. Houze
Lecture, Summer School on Severe and Convective Weather, Nanjing, 11-15 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
Types of feedbacks
•
•
•
•
Radiative
Water vapor
Latent heating
Momentum
Radiative feedback due to marine
stratus & stratocumulus cloud fields
is a major component of global
climate
Stratocumulus cloud radiative forcing
Deep Convection Feedbacks
These feedbacks are associated with
equatorial deep Cb and MCSs
Types of feedbacks
Heating and cooling processes in an MCS
Houze 1982
Water Budget of West African Mesoscale
Convective Systems
0
.13R
.37R
.41R
GATE
OCEANIC
CASE
West African
land case
1.17R
(Gamache
& Houze
1983)
(Chong & Hauser
1989)
.16R
.29R
.60R
.40R
Contributions to Total Heating by Convective Cloud System
Conv. LH
Strat. LH
DEPOSITION
Strat. Eddy
Conv. Eddy
Rad.
Details from Houze 1982
Simplified MCS Heating Profiles
Height (km)
Stratiform
Convective
Schumacher et al. 2004
Deg K/day
MCS Net Heating Profiles
70% stratiform
Height (km)
40% stratiform
0% stratiform
Schumacher et al. 2004
Deg K/day
TRMM precipitation radar rain amount divided
into convective & stratiform components
Total rain
Convective rain
Stratiform rain
Stratiform rain fraction
Schumacher & Houze (2003)
TRMM PR 1998-2000
annual precipitation, 0% stratiform, resting basic state
K/day
250 mb stream function, 400 mb heating
Schumacher et al. 2004
TRMM PR 1998-2000
annual precipitation, 40% stratiform, resting basic state
K/day
250 mb stream function, 400 mb heating
Schumacher et al. 2004
Radiative and water vapor feedbacks of
mesoscale convective systems
Think of the following schematic as a composite of the MCS over
its whole life cycle
Ac
As
Need to know structure
and composition of the
anvil clouds—from
observations, and
ultimately from models
After Houze et al. (1980)
Convective region water budget
Ac
As
Ac = Ccu - Rc - Ecd - CT
Stratiform region water budget
Ac
As
As = Csu + CT - Rs - Esd
Radiative and latent heating not independent!
ΔZAC
Ac
As
ΔXAC
 Ice water content of anvil clouds needs to be consistent with
overall water budget of the MCS
Momentum in MCSs
Circulation associated with idealized MCS
Midlevel
inflow
Low-level
inflow
Houze 1982
Low-level Inflow
Layer Model of Convection
Moncrieff 1992
TOGA COARE
Airborne Doppler Radar Observations of MCSs
25 convective region flights
Showed deep layer of inflow to updrafts
<0
Kingsmill & Houze 1999
Mid-level Inflow
Circulation associated with idealized MCS
Mid level
inflow
Low level
inflow
Houze 1982
Midlevel inflow can come from any direction
100 km
Idealized
radar echo pattern
Houze 1997
TOGA COARE
Airborne Doppler Observations of MCSs
25 stratiform region flights
Kingsmill & Houze 1999
Circulation Features of an MCS
Momentum transport in the
convective region
Buoyancy-produced pressure minimum in an
MCS observed by aircraft
Convective
Region
L
LeMone 1983
Pressure perturbation field in a simulated MCS
Precip.
Cloud
Yang & Houze 1996
Mesoscale Momentum Transport
Sizes of MCSs observed in TOGA COARE
“Superclusters”
Chen et al. 1996
Supercluster in a GCM
plan view
B
1000 km
A
Very
large
stratiform
region
1000 km
cross section
A
B
Moncrieff &
Klinker 1997
Example from TOGA COARE
BoB 1979
TEPPS
1997
EPIC
2001
(Dashed: No sounding network)
JASMINE
1999
Example of an MJO event in TOGA COARE
Example KR wave from TOGA
COARE
TOGA COARE radar data
strong westerly
phase
westerly
onset phase
Houze et al. 2000
TOGA COARE
Westerly wind component at 155°E
12-15 Dec 92
21-26 Dec 92
Westerly
jet
Westerly Onset
Strong Westerly
Houze et al. 2000
Strong westerlies
11 February 1993
reflectivity
Stratiform
radar echo
SW
NE
Doppler velocity
Ship Doppler radar
velocity field
superimposed on satellite
IR image
Downward
momentum
transport in
stratiform
region
Houze et al. 2000
Strong westerlies
Mesoscale model simulation of MCS in TOGA COARE strong westerly regime
Perturbation momentum
Mechem et al. 2004
Westerly Onset
Mesoscale model simulation of MCS in TOGA COARE westerly onset regime
Perturbation momentum
m/s
Mechem et al. 2004
Zonal momentum flux convergence in model
stratiform regions
Westerly
WesterlyOnset
onsetCase
- feedback
Strong
Westerly
Case
Strong
westerlies
+ feedback
Mechem et al. 2006
Summary of Cloud Feedbacks
• Radiative & Water Vapor
• Marine stratocumulus is major radiative feedback
• Anvils of MCSs produce both radiative & vapor
feedbacks
• Latent heating
• Stratiform regions make heating profile top heavy
• Momentum
• Convective region slope affects momentum transport
• Stratiform regions of large MCSs transport
momentum downward
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
Next
Diurnal variability
Clouds in tropical cyclones
This research was supported by
NASA grants NNX07AD59G, NNX07AQ89G, NNX09AM73G, NNX10AH70G, NNX10AM28G,
NSF grants, ATM-0743180, ATM-0820586,
DOE grant DE-SC0001164 / ER-6
Conceptual model of MCS
Houze et al. 1989
Moncrieff 1992
Strong westerlies
11 February 1993
reflectivity
Stratiform
radar echo
SW
NE
Doppler velocity
Ship Doppler radar
velocity field
superimposed on satellite
IR image
Downward
momentum
transport in
stratiform
region
Houze et al. 2000
Strong westerlies
11 February 1993
reflectivity
Stratiform
radar echo
SW
NE
Doppler velocity
Ship Doppler radar
velocity field
superimposed on satellite
IR image
Downward
momentum
transport in
stratiform
region
Houze et al. 2000
Westerly Onset
15 December 1992
Doppler Radial Velocity
0.5 km
Houze et al. 2000
Momentum Transport by Stratiform Region
Descent in Superclusters
++feedback
feedback
- -feedback
feedback
Houze et al. 2000