Effects of Topographic Smoothing on the Simulation of Winter Precipitation in High Asia

Water resources in High Mountain Asia are essential for hundreds of millions of people throughout southern Asia. Recently, numerous studies have made projections of future changes in the region's hydrology based on temperature and precipitation from global circulation models (GCM) under future climate scenarios. Although the potential benefit of such studies is immense, coarse grid-scale GCMs are unable to resolve High Asia's complex topography and thus have a biased representation of regional weather and climate. This manuscript investigates biases in the simulation of physical mechanisms that generate precipitation in High Asia in coarse topography experiments using the Weather Research and Forecasting model. Thirty-three extreme winter orographic precipitation events are simulated at fine atmospheric resolution with original 6.67km-resolution topography and modified 1.85°x1.25°-resolution topography, taken from a GCM. As with many modified topography experiments performed in other regions, the distribution of precipitation is highly dependent on first-order orographic effects, which dominate regional meteorology. However, we additionally demonstrate that topographic smoothing enhances the development of simulated extratropical cyclones, with significant impacts on regional orographic forcing and accumulated precipitation. Despite precipitation reductions of 33% over High Asia's highest ranges, due to reduced ascent on windward slopes, total precipitation over the study domain increased by an average of 9% in smoothed topography experiments on account of intensified extratropical cyclone dynamics and cross-barrier wind speeds. These findings identify an important source of bias in coarse-resolution simulated precipitation in High Asia, with important implications for the application of GCMs toward projecting future hydroclimate in the region.