Sensitivity of Simulated Summer Monsoonal Precipitation in Langtang Valley, Himalaya, to Cloud Microphsyics Schemes in WRF

A better understanding of regional-scale precipitation patterns in the Himalayan region is required to increase our knowledge of the impacts of climate change on downstream water availability. This study examines the impact of four cloud microphysical schemes (Thompson, Morrison, Weather Research and Forecasting (WRF) single-moment 5-class, and WRF double-moment 6-class) on summer monsoon precipitation in the Langtang Valley in the central Nepalese Himalayas, as simulated by the WRF model at 1 km grid spacing for a 10 day period in July 2012. The model results are evaluated through a comparison with surface precipitation and radiation measurements made at two observation sites. Additional understanding is gained from a detailed examination of the microphysical characteristics simulated by each scheme, which are compared with measurements using a spaceborne radar/lidar cloud product. Also examined are the roles of large- and small-scale forcings. In general, the schemes are able to capture the timing of surface precipitation better than the actual amounts in the Langtang Valley, which are predominately underestimated, with the Morrison scheme showing the best agreement with the measured values. The schemes all show a large positive bias in incoming radiation. Analysis of the radar/lidar cloud product and hydrometeors from each of the schemes suggests that “cold-rain” processes are a key precipitation formation mechanism, which is also well represented by the Morrison scheme. As well as microphysical structure, both large-scale and localized forcings are also important for determining surface precipitation. The good performance of the Morrison scheme is consistent with its double-moment prediction of every ice-phase hydrometeor, which is ideally suited to represent this mechanism. By contrast, the WRF double-moment scheme is the poorest performing scheme due to its double moment prediction of 'warm-rain processes' that did not occur, as well as excessive evaporation near the ground. A recommendation of this study is therefore that improved modelling of ice-phase processes is a critical criterion for a realistic representation of clouds and precipitation in the Langtang Valley region, and by extension throughout the wider Himalayan region.