Summer monsoon convection in the Himalayan region: terrain and land cover effects

Abstract

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 containing broad stratiform echo occur 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.

Improved understanding of an extreme rainfall event at the Himalayan foothills – a case study using COSMO

“…On the dry side, in Kabul, both temperature and dew point are well captured by the model with dry air around 700Á800 hPa (also observed westward near Jodhpur), a thin layer of relatively moist air at 600 hPa, and very dry air above 400 hPa. This is consistent with similar observations made by Medina et al (2010) near the western indentation, suggesting the presence of mid-level moist monsoonal air mass between the two sources of dry air.…”

“…The convergence of low-level moist air in the valley upstream to Ukhimath Tehsil and the development of a strong north-westerly dry wind above the ridges, pushes the potentially unstable air mass eastward over the protruding foothills of the Himalayas, which generates the necessary uplift to erode the lid and trigger convection. This orographic lifting of potentially unstable air acts as a trigger mechanism for deep convection over this region, which is consistent with and corroborates the findings from previous studies (Sawyer, 1947;Houze et al, 2007;Medina et al, 2010;Houze, 2012).…”

“…Along this cross-section, u e decreases with height up to 8 km, indicating a potentially unstable layer, while u increases with height everywhere. This setting is similar to the situation presented by Medina et al (2010) for a deep convective event near the western indentation during the monsoon season. The vertical structure of the atmosphere in the valley recesses upstream of Ukhimath Tehsil (D) is even better illustrated by the averaged vertical profiles of wind speed and vertical gradients of u and u e (Fig.…”

“…Houze et al (2007) present a schematic of the occurrence of deep convection indicated by strong Tropical Rainfall Measuring Mission (TRMM) radar echoes over the western sub-region (also documented by Barros et al, 2004;Zipser et al, 2006), produced by an inversion-capped lowlevel monsoonal flow, which accumulates instability via surface heating until this instability is released by orographically induced lifting. With backward trajectory analysis, Medina et al (2010) show that the typical deep convective events observed during monsoon in the western indentation of the Himalayas between the Karakoram and the Hindu Kush Mountains is usually preceded by this south-westerly monsoonal flow traversing the Thar Desert, while increasing the buoyancy of the moist air. They also show that orographic lifting by the foothills of the Himalayas triggers deep convection by removing the upper and lower lid inhibiting convection and releasing the deep potentially unstable layer.…”

Impact of the Darjeeling–Bhutan Himalayan front on rainfall hazard pattern

“…The interaction between large-scale circulation and the topography plays a crucial role in determining the rainfall pattern of the Eastern Himalaya (Anders et al 2006;Houze et al 2007;Murata et al 2007;Medina et al 2010;Goswami et al 2010;Romatschke and Houze 2011;Prokop and Walanus 2015). These studies have revealed two general rainfall gradients in this area.…”