Transcript 061031Boulder_ThompsonLecture_MonsoonV14_Houze.ppt

Monsoon Convection in the Himalayan Region
as seen by the TRMM Precipitation Radar
Robert A. Houze, Jr., Darren C. Wilton, and Bradley F. Smull
University of Washington
Thompson Lecture, NCAR, Boulder, 31 October 2006
Monsoon Convection in the Himalayan Region
as seen by the TRMM Precipitation Radar
Robert A. Houze, Jr., Darren C. Wilton, and Bradley F. Smull
University of Washington
Precipitation
Thompson Lecture, NCAR, Boulder, 31 October 2006
Goal
To gain insight into the physical mechanisms by which heavy monsoon
precipitation is produced
Approach
• Use data from the Precipitation Radar (PR) on the Tropical Rainfall
Measuring Mission (TRMM) satellite.
• Examine the three-dimensional structure of the storms producing
intense monsoon precipitation.
• Determine how the 3D echo structure varies in relation to details of the
Himalayan topography and proximity to surrounding oceans.
TRMM Precipitation Radar Data Set Used in
This Study
• June-September 2002, 2003
• 1648 Overpasses over Himalayan region
• Data specially processed at UW to optimize vertical structure analysis
Analysis Subregions
°N
Western
Subregion
Central
Subregion
Arabian
Sea
INDIA
Eastern
Subregion
Bay of
Bengal
°E
TRMM Satellite Instrumentation
l = 2 cm
Important! PR
measures 3D
structure of radar
echoes
Kummerow et al, 1998
Analysis of three-dimensional echo regions
Used TRMM algorithm for separating echoes into
stratiform & convective regions
 STRATIFORM identified by 2 criteria:
Existence of bright band
Lack of intense echo cores
 Non-stratiform is either CONVECTIVE or “OTHER”
Analysis of Convective Echo Cores
To study the vertical structure of convective
regions we first define 3D echo “cores”
• The TRMM Precipitation Radar data are provided in “bins” ~5
km in the horizontal and ~0.25 km in the vertical
• Echo cores are formed by contiguous bins (in 3D space) of
reflectivity values which exceed the threshold of 40 dBZ.
echo
core
3D radar echo bounded
by 40 dBZ contour
Western
Central
Deep Intense Cores
40 dBZ echo
> 10 km in height
Wide Intense Cores
40 dBZ echo
> 1000 km2 area
Broad Stratiform Echo
stratiform echo
> 50,000 km2
Eastern
Lightning frequency based on TRMM satellite
observations
Carlson et al. 1983
Sawyer 1947
A case of deep isolated 40 dBZ core
14 June 2002
10 meter level
200 mb level
A case of deep isolated 40 dBZ core
14 June 2002
0900 UTC
0930 UTC
A case of deep isolated 40 dBZ core
14 June 2002
0900 UTC
Deep cores over the Tibetan Plateau
14 July 2002
1227 UTC
Height of 40 dBZ cores by region
In western region-graupel particles
lofted to great
heights by strong
updrafts
A case of wide 40 dBZ echo core
22 July 2002
10 meter level
200 mb level
A case of wide 40 dBZ echo core
22 July 2002
A case of wide 40 dBZ echo core
22 July 2002
1300 UTC
1400 UTC
A case of wide 40 dBZ echo core
22 July 2002
1300 UTC
A typical case of wide 40 dBZ echo core with line
organization
2208 UTC
3 Sep 2003
A wide 40 dBZ echo core with squall-line
organization—rare!
2017 UTC
5 June 2003
A of wide 40 dBZ echo core with squall-line
organization—rare!
500 mb jet over
and parallel to the
Himalayas
10 meter level
500 mb level
5 June 2003
Cumulative Frequency
Horizontal area of 40 dBZ cores by region
In western
region—wide
convective areas
more frequent
Area (km2)
Analysis of Stratiform Echoes
Intraseasonal Variation of the Monsoon
Webster & Tomas 1997
Day 0:
8 mm/d
5N-5S
80-90E
39 events
1985-95
“Break”
“Active”
Broad stratiform case
11 Aug 2002
10 meter level
200 mb level
Broad stratiform case
11 Aug 2002
Broad stratiform case
11 Aug 2002
0252 UTC
Broad stratiform case
Upstream of mountains
0455 UTC
Size of stratiform precipitation area by
geographical region
Analysis of All the Reflectivity Data
Reflectivity data for 2 monsoon seasons
Relative frequency of occurrence
Reflectivity data for 2 monsoon seasons
Convection is
stronger &
deeper in west
Stratiform more
pronounced in
east
Reflectivity data for 2 monsoon seasons
Convection is
slightly deeper &
stronger over the
lowlands than
the foothills
Summary
• Strongest over lowlands
• West: “Deep” & “wide”
cores prone to occur just
upstream & over the
foothills, esp. in the
west, near confluence of
dry downslope &
maritime flows.
• Vertical cells
• Central: Get both deep
and wide cores, as in
west, but not as
frequent.
• Squall lines when jet
parallel to Himalayas
• East: Get mesoscale,
partially stratiform cloud
systems associated with
depressions over the
Bay of Bengal
• Mesoscale systems like
oceanic convection with
large stratiform regions
• Wide cores—amorphous
or parallel to mt. range
• Lots of lightning
• No squall lines
• Isolated cells over
plateau
• Get broad stratiform
regions associated with
depressions propagating
from equatorial region
Epilogue
•
What has this study accomplished?
1) Particular structure and organization of summer monsoon
convection over the subcontinent of South Asia
2) Behavior of highly convective clouds in a moist flow impinging
on a mountain barrier
•
What questions remain?
1) Why does the intense convection trigger just upstream of the
barrier?
2) In depressions, what are the relative roles of orography and
synoptic dynamics?
3) Can high-resolution models predict the observed structures?
Thanks