Effects of Large Mountain
Ranges on Extreme Rainstorms
Robert A. Houze, Jr.
with
S. Medina, U. Romatschke, K. Rasmussen,
S. Brodzik, D. Niyogi, and A. Kumar
Seminar, National Central University, Taiwan, 14 April 2011
Extreme mountain ranges
Rockies
Himalayas
Andes
Pakistan Floods—Summer 2010
"Almost 20 million people need shelter,
food and emergency care. That is more
than the entire population hit by the
Indian Ocean tsunami, the Kashmir
earthquake, Cyclone Nargis, and the
earthquake in Haiti—combined.”
Secretary-General Ban Ki-moon
August 2010
Pakistan Floods—Summer 2010
Types of rainstorms
Frontal systems
• midlatitude winter storms
Tropical cyclones
• hurricanes, typhoons
Convective storms
• local thunderstorms
• “mesoscale convective systems”
Example of a local thunderstorm
Viewed from the ground
Example of a local thunderstorm
Viewed from space
Oklahoma
100 km
Example of a
mesoscale convective system
Three MCSs
Radar echoes showing
the precipitation
in the 3 MCSs
Stratiform
Precipitation
Convective
Precipitation
1458GMT 13 May 2004
Idealized structure of a mesoscale
convective system
Stratiform
Radar echo
Convective
Stratiform
Houze 1989
The TRMM Satellite
TRM
M
Precipitation Radar
Low altitude, low inclination orbit
TRMM Precipitation Radar
 = 2 cm
Important!
Radar measures
3D structure of
radar echoes
>
Kummerow et al, 1998
The Himalayan region
A natural laboratory for
studying the behavior of
extreme convection near
a huge mountain range
Radar Identification of Extreme Rainstorms
Identify each contiguous 3D echo object
seen on radar
Convective component
Stratiform component
Extreme characteristic
Extreme characteristic
Contiguous 3D volume of
convective echo > 40 dBZ
Contiguous stratiform echo
with horizontal area > 50 000 km2
“Broad stratiform region”
Top height > 10 km
“Deep convective core”
Horizontal area > 1 000 km2
“Wide convective core”
Examples
Deep
Convective
Core
Stratiform
Wide
Convective
Core
Convective
Broad
Stratiform
Region
Climatology of
extreme convective
features
Based on 10 years of
data for the monsoon
season (JJAS)
Deep
Convective
Cores
Wide
Convective
Cores
Broad
Stratiform
Regions
Eastern Himalayan region
Rainstorms have
-massive stratiform regions
-which are enhanced by
airflow over the mountains
SUMMER MONEX
6 July 1979
850 mb wind
Houze and Churchill 1987
Microphysics
SUMMER MONEX
6 July 1979
20N
NOAA P3 Aircraft
Extensive
stratiform
precipitation
areas
Radar
20N
100 km
100 km
14N
14N
88E
92E
88E
92E
Houze and Churchill 1987
Example of a Bay of Bengal depression observed by
TRMM & simulated by WRF
WRF simulation
Surface wind
Total Rain
Medina et al. 2010
TRMM PR
WRF Simulation
(a)
Medina et al. 2010
WRF Simulation Summary
White -- 850 mb height
Yellow -- 500 mb vertical velocity
Medina et al. 2010
Climatology of
extreme convective
features
Based on 10 years of
data for the monsoon
season (JJAS)
Deep
Convective
Cores
Wide
Convective
Cores
Broad
Stratiform
Regions
Western Himalayan region
• Severe local storms
• Similarities to the region
east of the Rocky
Mountains
Carlson et al. 1983
Texas
Backward trajectories (HYSPLIT/NCEP)
2.5 km
1.0 km
Consistent with
Consistent
Sawyer
1947with
Sawyer 1947
Medina et al. 2010
WRF Simulation
Water vapor mixing
ratio at surface
Medina et al. 2010
CAPE
WRF Simulation
Isochrones of
integrated
hydrometeor
content
First cloud
formation
Medina et al. 2010
Hydrometeor
mixing ratio
(1 g/kg contour)
just after
convection
formed
TRMM PR Observations
WRF Simulation
Medina et al. 2010
The Pakistan Floods
Storms of the eastern type
occur in the west!
Normal 500 mb anomaly patterns
for South Asian rainstorms
Normal
Normal 500 mb anomaly patterns
for South Asian rainstorms
Normal
500 mb pattern 28 July 2010:
very abnormal
Time sequence…
700 mb wind (~3 km)
H
L
Water vapor anomaly
500 mb wind
Rain
L
L
L
A humid environment was
created in a dry place
200
200
Pressure (mb)
Flood case
Bengal case
Typical
environment
500
1000 0
500
50
Relative humidity (%)
100
1000
-40
0
Temperature (C)
40
Broad stratiform precipitation occurred
over the mountains of Pakistan  Floods!
Broad stratiform!
More
about the
Pakistan
floods
coming in
next BAMS
East of the Andes
Another region of intense
convection…
Is it like convection near the
Rockies and Himalayas?
The geography
Average airflow
conditions for
storms with
wide convective
cores
Vertical air motions
up down
Low-level winds
moist
unstable
Example of
triggering over
the Sierra
Cordoba range
What the storm
looked like on
the TRMM
radar
Conclusions
• Large mountain ranges affect extreme
convective rainstorms by:
•
•
•
•
Channeling low-level moisture
Producing capping of the moist layer
Triggering convection
Enhancing the stratiform components of
mesoscale convective systems
• Processes occur in different combinations
• Understanding them can help understand
extreme events such as the Pakistan floods
End
This research was supported by
NSF grant ATM-0820586 and NASA grant NNX10AH70G