1. The effect of terrain and land surface on summer monsoon convection in the Himalayan region Socorro Medina, Robert Houze, Anil Kumar, and Dev Niyogi 13 th Conference on Mountain Meteorology), Whistler, BC, Canada, 12 August 2008

2. Orographic Precipitation in cold and warm climates ALPS – MAP (1999) OR CASCADES – IMPROVE-2 (2001) HIMALAYAS

3. Terrain gradients Land-ocean contrast Land cover gradients Snow/Ice Tundra Wetland Forest Irrigated crop Crop Savanna Shurb/Grass Dryland/crop Grass Shurb Barren Thar Desert Ganges Delta

4. 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

5. 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

6. Dominant type of systems Deep and wide convective systems Broad stratiform echoes (embedded in convective systems)

7. Wide convective system in western indentation 3 September 2003

8. Domain 1 (dx = 9 km)Domain 2 (dx = 3 km) Accumulated precipitation and terrain INDIA PAKISTAN HIMALAYAS HINDU KUSH

9. Evaluation: 3D Reflectivity structure (~22 UTC 3 Sep [~03 LST 4 Sep]) OBSERVATION (TRMM-PR) Horizontal cross sections at 4 km Vertical cross sections along black line Vertical cross sections along red line SIMULATION dBZ 0 100 200 Distance (km) Height (km) 0 8 16 0 100 200 Distance (km) Height (km) 0 8 16 0 125 250 Distance (km) Height (km) 0 8 16 0 125 250 Distance (km) Height (km) 0 8 16

10. 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)

11. 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 1.0 AGL km 3.5 AGL km Valid: 18 UTC 3 Sep (23 LST) Forecast : 0 h End time: 18 UTC 3 Sep (23 LST) Elapsed period between markers: 24 h

12. HYPOTHESIS: High surface sensible heat flux as low-level air moves over Thar Desert NCEP time series 

13. HYPOTHESIS: Convection triggered over foothills TERRAIN AND COLUMN INTEGRATED PRECIPITATION HYDROMETEORS (10 mm) TOTAL PRECIP. MIXING RATIO Valid: 19 UTC 3 Sep (00 LST). Forecast : 1 h N 6.0 g kg -1

14. 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