WRF-based simulation of an extreme precipitation event over the Central Himalayas: Atmospheric mechanisms and their representation by microphysics parameterization schemes

Floods and landslides represent a major risk for the Himalayan region of Nepal and the increasing frequency of intense precipitation events during recent decades leads to an escalating exposure to hazards. A better understanding of the dynamics causing extreme rainfall events is crucial in order to improve their prediction and to minimize the associated impacts. Against the background, that the meso- and micro-scale characteristics of high intensity precipitation events in the Himalayas are not yet well understood, we investigate an extreme precipitation event that led to massive flooding and landslides in western Nepal on 14-15 Aug 2014. During the event, a maximum precipitation amount of 528 mm/24 h was observed at Chisapani station. In order to simulate the atmospheric dynamics during the event, we employ the Weather Research and Forecasting (WRF) model at convection-permitting scale. Aiming at the identification of a suitable model configuration, we conduct sensitivity experiments for 9 cloud microphysics schemes and evaluate each of them against dense network of 222 precipitation stations by means of an object-based diagnostic evaluation (MODE) technique. Subsequently, the meso- and micro-scale dynamics are investigated considering relatively better performing microphysics schemes as a reference. Our results show that the essential features of the extreme event are well simulated by WRF, although the exact location and intensity varies from scheme to scheme. Two third of the considered microphysics schemes are generally able to capture the event, with the Thompson scheme showing the best agreement with observations. The investigation of the regional circulation reveals, that the event was associated with a southward expansion of an upper-level westerly trough and a contemporaneous northward migration of the low-level monsoonal trough towards Himalayan foothills. The interaction of these two systems intensified the low-level moisture fluxes from the Arabian Sea towards the Himalayan foothills. The high atmospheric moisture content, in combination with strong sensible heating of the Indian Plains and the Himalayan foothills provoked a destabilization of the atmosphere. The strong cross barrier flow further initiated the meso-scale convective systems due to the orographic uplift at the windward Himalayan slopes. The above mentioned synoptic conditions can be considered as pre-indicators for high impact precipitation events and thus indicate a certain degree of predictability. However, our results show that the selection of an appropriate microphysics scheme is crucial for the reliable prediction of intense convective rainfall events by means of convection permitting scale NWP models.