The effect of terrain and land surface on summer monsoon convection in the Himalayan region

As the southwesterly moist monsoonal flow progresses from the Arabian Sea and the Bay of Bengal toward the Indian subcontinent, it impinges upon the Himalayan Mountain Range. However, there are considerable differences in the characteristics of the precipitating systems observed on the western and eastern sides of the Himalayas, as revealed by a statistical study of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) echoes (Houze et al. 2007). Near the western indentation of the terrain, deep convective echoes (40 dBZ echo > 10 km in height) and wide convective echoes (40 dBZ echo > 1000 km² in area) dominate, while  on the eastern side, broad stratiform echoes (> 50,000 km² in area) occur in connection with Bay of Bengal depressions. The precipitating systems appear to be controlled by surface-atmosphere processes, in particular by 1) the interaction between the flow and the orography and 2) surface fluxes of sensible and latent heat. The surface fluxes depend on the land surface characteristics, which has large gradients upstream of the elevated terrain, where the west side is characterized by barren and sparsely vegetated landscape and the east side is dominated by a mixture of forests, crops, and wetlands.

This study investigates the role that the terrain and the land surface fluxes play in controlling, triggering, and sustaining mesoscale convection associated with the summer monsoon in the Himalayan region. In this investigation we analyze high-resolution (3-km in the horizontal) numerical Weather Research and Forecasting (WRF) simulations of archetypical precipitating systems in both the eastern and western portions of the Himalayan region. The horizontal scale of isolated deep convective systems was too small to be captured well by the 3-km resolution WRF simulation; however, the model was able to capture wide convective and broad stratiform systems.

Analysis of WRF output of a typical wide convective system reveals that this type of event occurs as low-level, moist monsoonal air from the Arabian Sea is capped by a layer of dry downslope flow off the Afghan mountains. This configuration is similar to what is often observed in the spring over the U.S. Great Plains, when a dry-line forms between moist flow from the Gulf of Mexico and dry flow off the Mexican Plateau (Carlson et al. 1983). As the low-level moist air moves over the hot and arid western Indian subcontinent, it accumulates instability via sensible heat fluxes. The CAPE values on the moist side of the dry-line are very high (> 3000 J/kg). However, the capping lid prevents release of instability. The instability is released over small peaks of the terrain when the low-level flow reaches the lower Himalayan foothills. In this case, the terrain plays three main roles in modulating convection: 1) The west concave indentation of the terrain prevents the low-level, moist flow from continuing moving northward and allows low-level moisture to build up; 2) The elevated layer of dry, warm air from the Afghan mountains caps the low-level, monsoonal flow, allowing buoyancy to build up; 3) The convection is triggered at the small peaks of the terrain (~0.5 km) by orographic lifting.

            A WRF simulation of a broad stratiform system occuring in association with a Bay of Bengal depression suggests that in this type of event, the synoptic scale depression provides persistent, moist, cross-barrier, low-level flow. The moisture source is the Bay of Bengal and the Ganges Delta to a lesser degree. The low-level flow is sub-saturated and potentially unstable. When it reaches the Himalayan foothills, it is lifted to saturation and the instability is released. Large upward motions produce intense and deep convective precipitation echoes on the foothills. As the system ages, the echoes are advected downstream, the precipitation becomes wide-spread and it takes a stratiform character. In this case, the terrain affects the convection in the following ways: 1) The east concave indentation of the terrain prevents the low-level, moist flow from continuing moving northward and allows low-level moisture to build up; 2) The convection is triggered as potentially unstable low-level flow is lifted over the foothills, releasing instability; 3) As the system ages, the precipitating echoes become stratiform and are advected downstream, toward the eastern indentation of the terrain, where orographic lifting further enhances the precipitation.