Mesoscale Convective Systems:
Recent Observational and Diagnostic Studies
Robert Houze
Department of Atmospheric Sciences
University of Washington
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?
Why do tropical and midlatitude
MCSs look different?
Radar
reflectivity
Houze et al. 1989, 1990
Tropical &
midlatitudes
“Symmetric”
Midlatitudes
(later stages)
“Asymmetric”
Strat.
Conv.
Skamarock et al. 94
No Coriolis
Coriolis
MCV
Symmetric
(Tropics & midlatitudes)
Asymmetric
(Midlatitudes)
Parcel vs. layer lifting in an MCS
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 0.5-4.5 km
z
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
Moncrieff & Klinker 1997
A
B
1000 km
plan view
1000 km
cross section
A
B
Is rear inflow really from the rear?
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
Factors determining the size of MCS
ICAPE, sustainability, diurnal cycle
Sizes of MCSs observed in TOGA COARE
“Super Convective Systems”
(SCS)
Chen et al. 1996
Kingsmill & Houze 1999
Examples
of TOGA
COARE
MCSs
Satellite IR
overlaid
with A/C
radar
100 km
Yuter & Houze 1998
Convective echo
% of grid
CS map
Stratiform echo
Mean IR temp (K)
Satellite IR
% of grid
y
(km)
x (km)
Statistics for all TOGA COARE satellite/radar comparisons
Yuter &
Houze
1998
Percent of 24
km square grid covered by A/C radar echo in all the MCS
Statistics for all TOGA COARE satellite/radar comparisons
Yuter & Houze 1998
Portion of 240
km scale grid covered by convective radar echo
Schumacher & Houze 2003
TRMM Precipitation radar:
% of 2.5 deg grid covered by stratiform radar echo
Annual Average
Factors determining the movement of MCS:
Waves in the environment, cold pool dynamics
Nakazawa 1988
INTRASEASONAL
ENSEMBLE VARIATION
SUB-ENSEMBLE
MESOSCALE CONVECTIVE SYSTEM
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
Serra & Houze 2002
TEPPS—East Pacific ITCZ
Ship radar
data
Easterly wave
and cold
pool propagation
hard
to distinguish
JASMINE: May 1999
40N
NOAA Ship
R.H. Brown
equator
60E
100E
JASMINE IR sequence
(courtesy P. Zuidema)
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
Carbone et al. 2002
WSR88-D
radar data
over U.S.
in time/
longitude
format
Conclusions
Coriolis effect explains why midlatitude MCSs exhibit
asymmetry and develop MCVs as they evolve—and why
tropical MCSs don’t have asymmetry
Parcel lifting gives way to layer lifting in mature MCSs
when potentially unstable inflow air becomes
moistened—circulations become mesoscale!
Midlevel inflow enters stratiform regions from various
directions—controlled by environment shear
Max size of MCSs related to sustainability of low-level
moist inflow—get biggest systems over oceans and with
LLJs
MCSs motion may be determined by waves propagating
through the environment—gravity waves, inertio-gravity
waves,…