Snowpack Changes in the Sierra Nevada: High-Resolution Projections for the End of 21st Century

Friday, 19 December 2014
Daniel Walton, Alexander D Hall, Fengpeng Sun, Neil Berg and Marla Ann Schwartz, University of California Los Angeles, Los Angeles, CA, United States
The snowpack of the Sierra Nevada plays an important role as a natural reservoir for the surrounding region, storing water during the wet season and releasing it later in the year. Changes to the snowpack between the 1981-2000 and 2081-2100 periods are investigated using dynamical downscaling with WRF v3.2 coupled to the NOAH-MP land surface model, at 3km resolution. First, a baseline run is performed in which WRF is forced by North American Regional Reanalysis (NARR) for the 1981-2000 period. Based on a comparison between this baseline simulation and station data from the California Department of Water Resources, WRF is shown to have high model skill in reproducing snow water equivalent (SWE) variability. Next, five future runs are performed in order to simulate how that same baseline period would transpire if the mean climate were altered to reflect the climate change signal found in five CMIP5 GCMs. All five runs show an overall decline in April 1st SWE, but some runs show increased SWE at high elevations. SWE changes are influenced by competing factors including reduced precipitation falling as snow, increased total precipitation, and enhanced melting. Snow at lower elevations is highly temperature sensitive, so lower elevations experience higher fractional SWE losses. Meanwhile, higher elevations may still be below freezing under a warmer climate, so increased precipitation can lead to increased snowfall. The largest SWE losses occur at intermediate elevations that are cold enough to maintain moderate SWE levels during the baseline, but are still sensitive to temperature changes.