Vertical

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The effect of terrain and land surface on summer
monsoon convection in the Himalayan region
Socorro Medina, Robert Houze, Anil Kumar, and
Dev Niyogi
13th Conference on Mountain Meteorology),
Whistler, BC, Canada, 12 August 2008
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Orographic Precipitation in cold and warm
climates
ALPS MAP (1999)
OR CASCADES IMPROVE-2 (2001)
HIMALAYAS
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• Terrain gradients
• Land-ocean contrast
• Land cover gradients

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OBJECTIVE

• Observational studies (Sawyer et al. 1947, Houze
  et al. 2007) proposed hypotheses on how monsoon
  convection forms
• Objective Test hypotheses (in following slides)
  using model simulations

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Model/data used

• Weather Research and Forecasting (WRF v2.1.1)
  model with single-moment bulk microphysical
  parameterization with 6 water substances
• Complemented with NCEP data

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Dominant type of systems
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Wide convective system in western indentation3
September 2003
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Accumulated precipitation and terrain
Domain 1 (dx 9 km)
Domain 2 (dx 3 km)
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Evaluation 3D Reflectivity structure (22 UTC 3
Sep 03 LST 4 Sep)
OBSERVATION (TRMM-PR)
SIMULATION
Horizontal cross sections at 4 km
Vertical cross sections along black line
Vertical cross sections along red line
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HYPOTHESIS Dry line
SURFACE DEW POINT DEPRESSION AND 2 AND 4 KM
TERRAIN CONTOURS
Valid 18 UTC 3 Sep (23 LST) Forecast 0 h (1
h before convection initialization)
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HYPOTHESIS Moist low-level flow from Arabian
Sea, dry flow aloft from Tibetan or Afghan
mountains
SURFACE MIXING RATIO (g/kg)
NOAA HYSPLIT (NCEP FNL) BACKWARD TRAJECTORIES
Valid 18 UTC 3 Sep (23 LST) Forecast 0 h
End time 18 UTC 3 Sep (23 LST) Elapsed period
between markers 24 h
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HYPOTHESIS High surface sensible heat flux as
low-level air moves over Thar Desert
NCEP time series
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HYPOTHESIS Convection triggered over foothills
TOTAL PRECIP. MIXING RATIO
TERRAIN AND COLUMN INTEGRATED PRECIPITATION
HYDROMETEORS (10 mm)
Valid 19 UTC 3 Sep (00 LST). Forecast 1 h
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CONCLUSIONS WIDE CONVECTIVE CASE

• Moist low-level flow from Arabian Sea heated by
  passage over Thar Desert
• Western indentation of barrier allows low-level
  moisture and buoyancy to build up
• Elevated layer of dry, warm air from Afghan
  mountains caps the moist low-level flow
• Convection triggered by orographic lifting over
  the small peaks