Accumulation and melt dynamics of snowpack from a multi - resolution regional 1 climate model in the central Sierra Nevada

15 16 The depth and timing of snowpack in the Sierra Nevada Mountains are of fundamental 17 importance to California water resource availability, and recent studies indicate a shift 18 towards earlier snowmelt consistent with projected impacts of anthropogenic climate 19 change. In order for future studies to assess snowpack variability on seasonal to 20 centennial timescales, physically-based models of snowpack evolution at high spatial 21 resolution must be improved. Here, we evaluate modeled snowpack accuracy for the 22 central Sierra Nevada in the Weather Research and Forecasting regional climate model 23 coupled to the Noah land surface model. A simulation with nested domains at 27, 9, and 24 3 km grid spacings is presented for November 2001-July 2002. Model outputs are 25 compared with daily snowpack observations at 41 locations, air temperature at 31 26 locations, and precipitation at 10 locations. Comparison of snowpack at different 27 resolutions suggests that 27 km simulations substantially underestimate snowpack, while 28 9 km and 3 km simulations are closer to observations. Regional snowpack accumulation 29 is accurately simulated at these high resolutions, but model snowmelt occurs an average 30 of 22-25 days early. Some error can be traced to differences in elevation and observation 31 scale between point-based measurements and model grid cells, but these factors cannot 32 explain the persistent bias towards early snowmelt. A high correlation between snowmelt 33 and error in modeled surface air temperature is found, with melt coinciding 34 systematically with excessively cold air temperatures. One possible source of bias is an 35 imbalance in turbulent heat fluxes, erroneously warming the snowpack while cooling the 36 surface atmosphere. 37 38