Mesoscale Convective Systems:
Recent Observational and Diagnostic Studies
Robert Houze
Department of Atmospheric Sciences
University of Washington
10th Conf. on Mesoscale Meteorology, Portland, OR, June 23-27 2003
DEFINITION
Mesoscale Convective System (MCS)
A cumulonimbus cloud system that produces a
contiguous precipitation area ~100 km or
more in at least one direction
Questions
Why do tropical and midlatitude MCSs look different?
Does layer lifting occur in a mature MCS?
Is rear inflow really from the rear?
What controls the size of MCSs?
What controls the movement of MCSs?
Radar
reflectivity
Houze et al. 1989, 1990
Tropical &
midlatitudes
“Symmetric”
Midlatitudes
(later stages)
“Asymmetric”
Strat.
Conv.
Skamarock et al. 1994
No Coriolis
Coriolis
Symmetric
(Tropics & midlatitudes)
Asymmetric
(Midlatitudes)
Questions
Why do tropical and midlatitude MCSs look different?
Does layer lifting occur in a mature MCS?
Is rear inflow really from the rear?
What controls the size of MCSs?
What controls the movement of MCSs?
Parcel viewpoint
Zipser 1977
Crossover
Zone
Layer viewpoint: Bryan and Fritsch 2000
MAUL
“Slab” or Layer Overturning
Layer viewpoint: Kingsmill & Houze 1999
TOGA COARE
Airborne Doppler Observations of MCSs
Convective region flights
Note!
 e
0
z
0.5-4.5 km
Moncrieff & Klinker 1997
A
B
TOGA COARE
convection in
a GCM
with ~80 km
resolution
1000 km
plan view
1000 km
cross section
A
B
Pandya & Durran 1996
Mean heating
in convective
line
Horizontal
wind
gravity wave
response to heating
Questions
Why do tropical and midlatitude MCSs look different?
Does layer lifting occur in a mature MCS?
Is rear inflow really from the rear?
What controls the size of MCSs?
What controls the movement of MCSs?
Diversity of stratiform structure: Parker & Johnson 2000
PATTERNS OF
EVOLUTION OF
STRATIFORM
PRECIPITATION
IN MIDLATITUDE
SQUALL LINES
Kingsmill & Houze 1999
Documented airflow
shown by airborne Doppler in
TOGA COARE MCSs
Stratiform region flights
0°
C
JASMINE: Ship radar, Bay of Bengal, 22 May 1999
11
Height (km)
Refl.
0
0
11
Horizontal Distance (km)
192
Reflectivity
1.5 km level
100 km
Height (km)
Radial
Velocity
0
0
Horizontal Distance (km)
192
Radial Velocity
3.5 km level
90 km
JASMINE: Ship radar, Bay of Bengal, 22 May 1999
12
Height (km)
Refl.
0
0
12
Horizontal Distance (km)
192
Reflectivity
1.5 km level
100 km
Height (km)
Radial
Velocity
0
0
Horizontal Distance (km)
192
Radial Velocity
3.5 km level
90 km
Questions
Why do tropical and midlatitude MCSs look different?
Does layer lifting occur in a mature MCS?
Is rear inflow really from the rear?
What controls the size of MCSs?
What controls the movement of MCSs?
Sizes of MCSs observed in TOGA COARE
“Super Convective Systems”
(SCS)
Chen et al. 1996
Yuter & Houze 1998
Percent of 24 km square grid covered by A/C radar echo in all the MCS
All TOGA COARE satellite/radar comparisons
Precipitation
%
Convective
%
Stratiform
%
Yuter & Houze 1998
Percent of 240 km square covered by A/C radar echo in all the MCS
All TOGA COARE satellite/radar comparisons
Hypothesis:
The size of the MCS
is determined by the environment’s
ability to sustain an ensemble of
convection over time.
Question:
What factors control and limit
sustainability?
Height (m)
Kingsmill & Houze 1999: TOGA COARE a/c soundings
Schumacher & Houze 2003
TRMM Precipitation radar:
% of 2.5 deg grid covered by stratiform radar echo
Annual Average
Stratiform Rain Fraction
Inference:
Sustainability promoted by moist boundary layer that is
not interrupted by the diurnal cycle
Questions
Why do tropical and midlatitude MCSs look different?
Does layer lifting occur in a mature MCS?
Is rear inflow really from the rear?
What controls the size of MCSs?
What controls the movement of MCSs?
Traditional view:
Cold pool dynamics
Recent studies:
Waves in environment
IN TOGA
COARE
MCSs moved
Chen, Houze,
& Mapes 1996
individually
Analyzed
IR data
3°N-10°S
208°K threshold
with wave
much of the
time
Time (day)
12
13
14
15
Longitude
A/C
flights
on
12-14
Dec
JASMINE: May 1999
40N
NOAA Ship
R.H. Brown
equator
60E
100E
Webster et al. 2002
IR over Bay of Bengal during JASMINE
Ship track
5
10
15
20
May 1999
25
30
Mapes et
al. (2002)
West
Coast of
South Am.
Gravity
Wave
hypothesis
JASMINE
MCS
JASMINE
MCS
Conclusions
Coriolis effect explains why midlatitude MCSs exhibit
late-stage asymmetry not observed in the tropics.
Layer lifting occurs in mature MCSs, possibly as a
gravity wave response to the net heating in the
convective region.
Midlevel inflow enters stratiform regions from various
directions—controlled by environment wind.
Max size of MCSs related to sustainability of low-level
moist inflow—get biggest systems over oceans and with
LLJs
Movement of an individual MCS may be in part
determined by waves propagating through the
environment—gravity waves, inertio-gravity waves,…
Layer viewpoint: Mechem, Houze, & Chen 2002
14
TOGA COARE
23 Dec 92
12
10
Z (km)
150
8
6
Y (km)
100
4
50
2
150
200
X (km)
250
0
150
200
X (km)
250
Yuter & Houze 1998
Convective echo
% of grid
CS map
Stratiform echo
Mean IR temp (K)
Satellite IR
% of grid
y
(km)
x (km)
Nakazawa 1988
INTRASEASONAL
ENSEMBLE VARIATION
SUB-ENSEMBLE
MESOSCALE CONVECTIVE SYSTEM
JASMINE IR sequence
(courtesy P. Zuidema)
Serra & Houze 2002
TEPPS—East Pacific ITCZ
Ship radar
data
Easterly wave
and cold
pool propagation
hard
to distinguish
Carbone et al. 2002
WSR88-D
radar data
over U.S.
in time/
longitude
format
Examples
of TOGA
COARE
MCSs
Satellite IR
overlaid
with A/C
radar
240 km