Simulation of Historical and Future Precipitation over the Wasatch

Abstract:

This research presents the results of a retrospective simulation of precipitation over Utah, focused on the Wasatch Mountains spanning calendar years 1985-2010. Here WRF is used to downscale CFSR within three nested domains, with the 4-km innermost domain coupled with a slab lake model developed by Strong et al. (2014) to better evaluate the regional effects of the Great Salt Lake. Results are verified using SNOTEL and PRISM data, and while noting the well-documented SNOTEL limitations, this simulation properly depicts the spatial and temporal distribution of precipitation over the Wasatch Mountains. Noted biases are explored (WRF minus SNOTEL), including the apparent relationship between the sign of significant daily bias and mean atmospheric properties. Large positive differences between the model and SNOTEL mean daily totals correlate with warm, moist, quasi-zonal flow, while relatively cool, dry, trough-type regimes express the dry tail of the bias distribution. These conclusions sugget the role of processes internal to WRF and the applied schemes in the generation of the largest daily biases which then accumulate to monthly and seasonal departures from observed climatology. Additionally, preliminary results are presented from future projections, which apply this same WRF-downscaling methodology to CCSM at end of the current century. These results indicate noteworthy changes in the median April 1 Snow Water Equivalent by end of century across the Wasatch Mountains. Compared to retrospective runs, future projections indicate a net reduction in SWE of approximately 15% accompanying a modest (<10%) increase at very high elevation exceeding approximately 8500 feet.