Changes in Extreme Orographic Precipitation in the Interior Western United States By ~2050: Insights from a 30-Year Convection-Permitting Simulation

Data from a >30-year, high-resolution (4 km) regional climate simulation (10/1981-recent) covering the Interior Western United States (IWUS) are used to examine extreme precipitation characteristics, in particular over the IWUS mountains. The Weather Research and Forecasting (WRF) model is used at a high spatial resolution in order to capture the IWUS complex terrain environment and the high spatial variability of atmospheric and land surface variables. The simulation, driven by the NCEP Climate Forecast System Reanalysis (CFSR), is convection-permitting and uses the Noah Multi-Physics land surface model. This IWUS simulation has been validated in terms of surface temperature, snowpack (SWE), and especially orographic precipitation (Wang et al. 2018; Jing et al. 2018). It has been repeated for a 30 year period centered at anno 2050, driven by CFSR plus perturbations given by a multi-model ensemble mean from the ICCP RCP 8.5 climate change scenario, using the pseudo-global warming (PGW) technique. Climatic changes in precipitation rate distribution and return frequency of extreme precipitation events are examined. The comparison reveals a clear shift towards less frequent light precipitation and more frequent heavy precipitation. There are some interesting geographical and seasonal patterns in the projected change in extreme precipitation. In general, Return periods of extreme precipitation events are reduced by a factor of two or more indicating extreme precipitation events would occur twice as often in the future climate.