What Controls Wintertime Precipitation Distribution Across a Mountain Range? Insights from Regional Climate Simulations in the Interior Western US

There are several high-resolution (4 km) precipitation datasets covering the western US: NCEP Stage IV does poorly over mountains because of lack of low-level radar coverage. PRISM is available only at time scales of a month or longer, since it is a statistically-based dataset using gauges and SNOTEL data. Orographic precipitation in a multi-year high-resolution regional climate simulation (Wang et al. 2016) compares so well against SNOTEL that it can be used to study orographic precipitation distribution for individual storms and to question PRISM's finescale precipitation distribution. Regarding the latter, PRISM is good in areas near SNOTEL sites and gauges but questionable at higher elevation. Generally, the absolute bias between WRF and PRISM correlates with terrain elevation. Regarding the former, an isolated mountain range (the Wind River Range in Western Wyoming) is selected to analyze the ambient factors controlling the wintertime orographic, using the hourly outputs of WRF simulation. The mountain crest orients NW-SE, and the prevailing low-level wind direction during winter storms is from the SW, with a narrow azimuth range. The results show cross-barrier wind speed has a strong role in determining the distribution and amount of precipitation. Stability is important in controlling the precipitation in the lee: low stability tends to result in more precipitation in the lee. Cloud base height, which relates to low-level temperature and humidity, also plays important role in determine the surface precipitation amount and distribution.