During the Asian summer monsoon, convection occurs frequently near the
Himalayan foothills. However, the nature of the convective systems varies
dramatically from the western to eastern foothills. The analysis of high-resolution
numerical simulations and available observations from two case-studies and of
the monsoon climatology indicates that this variation is a result of region-specific
orographically modified flows and land surface flux feedbacks.
Convective systems containing intense convective echo occur in the western region
as moist Arabian Sea low-level air traverses desert land, where surface flux of sensible
heat enhances buoyancy. As the flow approaches the Himalayan foothills, the soil
may provide an additional source of moisture if it was moistened by a previous
precipitation event. Low-level and elevated layers of dry, warm, continental flow
apparently cap the low-level moist flow, inhibiting the release of instability upstream
of the foothills. The convection is released over the small foothills as the potentially
unstable flow is orographically lifted to saturation.
Convective systems containingbroad stratiform echooccur in the eastern Himalayas
in association with Bay of Bengal depressions, as strong low-level flow transports
maritime moisture into the region. As the flow progresses over the Bangladesh
wetlands, additional moisture is extracted from the diurnally heated surface.
Convection is triggered as conditionally unstable flow is lifted upstream of and
over the foothills. The convective cells evolve into mesoscale convective systems
(MCSs) with convective and stratiform areas. The MCSs are advected farther into
the Himalayan eastern indentation, where orographic lifting enhances the stratiform
precipitation. Copyright
c 2010 Royal Meteorological Society