Simulation of a Flash Flooding Storm at the Steep Edge of the Himalayas*

A flash flood and landslide in the Leh region of the Indus Valley in the Indian state of Jammu and Kashmir on 5–6 August 2010 resulted in hundreds of deaths and great property damage. Observations have led to the hypothesis that the storm, which formed over the Tibetan Plateau, was steered over the steep edge of the plateau by 500-hPa winds and then energized by the ingestion of lower-level moist air, which was approaching from the Arabian Sea and Bay of Bengal and rose up the Himalayan barrier. A coupled land surface and atmospheric model simulation validates this hypothesized storm scenario, with themodel storm taking the form of a traveling mesoscale squall line with a leading convective line, trailing stratiform region, and midlevel inflow jet. In this region, the development of a mesoscale storm over high terrain is highly unusual, especially one in the form of a propagating squall line system. This unusual storm occurrence and behavior could serve as a warning sign in flash flood prediction. The coupled atmosphere and land surface model showed that the excessive runoff leading to the flood and landslide were favored by the occurrence of this unusual meteorological event coinciding temporally and spatially with favorable hydrologic conditions. Additionally, the model simulations showed that previous rainstorms had moistened the soil during the entire season and especially over the few days leading up to the Leh flood, so the normally arid mountainsides were likely not able to rapidly absorb the additional rainfall of the sudden 5 August squall line.