Climate change projection of snowfall in the Colorado River Basin using dynamical downscaling

Recent observations show a decrease in the fraction of precipitation falling as snowfall in the western United States. In this work we evaluate a historical and future climate simulation over the Colorado River Basin using a 35 km continuous 111 year simulation (1969-2079) of the Weather Research and Forecasting (WRF) regional climate model with boundary forcing from the Hadley Centre for Climate Prediction and Research/Met Office's HadCM3 model with A2 emission scenario. The focus of this work is to (1) evaluate the simulated spatiotemporal variability of snowfall in the historical period when compared to observations and (2) project changes in snowfall and the fraction of precipitation that falls as snow during the 21st century. We find that the spatial variability in modeled snowfall in the historical period (1981-2005) is realistically represented when compared to observations. The trends of modeled snowfall are similar to the observed trends except at higher elevations. Examining the continuous 111 year simulation, we find the future projections show statistically significant increases in temperature with larger increases in the northern part of the basin. There are statistically insignificant increases in precipitation, while snowfall shows a statistically significant decrease throughout the period in all but the highest elevations and latitudes. The fraction of total precipitation falling as snow shows statistically significant declines in all regions. The strongest decrease in snowfall is seen at high elevations in the southern part of the basin and low elevations in the northern part of the basin. The regions of most intense decreases in snow experience a decline of approximately 50% in snowfall throughout the 111 year simulation period. The regions of strongest declines in snowfall roughly correspond to the region of migration of the zero degree Celsius line and emphasize snowfall dependence on both altitude and latitude.