Impacts of terrain resolution on an extreme snowstorm process over the Southern Tibetan Plateau: Spatial distribution and cloud microphysical processes

Precipitation is an essential component of the water cycle over the Tibetan Plateau (TP) in cold seasons. Investigating the impacts of various terrain resolutions in simulations on the precipitation distribution and rain-snow phase transition on the TP helps to understand the sources of simulation error and contributes to assessment and improvement. In this study, three types of terrain resolutions were used to conduct simulation experiments (namely WRF30s, WRF2m and WRF5m) on a large-scale extreme precipitation process over the TP under the influence of a storm over the Bay of Bengal in October 2008 near the Himalayas and the Yarlung Zangbo Grand Canyon, which serve as representative regions influenced by northward moisture. The results indicate that the terrain resolutions affect the precipitation amount and phase distribution. On the north of the Himalayas, a snowfall-dominated “phase bias” based on “wet bias” is observed above 3 km, which strengthens as the terrain resolution reduces, relating to the stronger water vapor flux transport and larger cold bias resulting from the terrain smoothing. The Bomi station, which is located in the canyon, undergoing a snow-to-rain phase transition influenced by the storm received further attention. The study not only shows the cloud microphysical characteristics of the precipitation on the TP under the influence of storms, but further reveals the snow melting phase transition weakening, manifested as the increase of snowfall accompanied by the decrease of terrain resolution. Further detailed analysis of cloud microphysical process in Bomi shows that the latter is related to the decrease melting rates of snow and graupel caused by the compression of the melting layer when the terrain near canyon increases with terrain resolution decrease. Consequently, the distribution and phase transition of precipitation on the TP induced by terrain resolutions requires attention during the simulation.