Precipitation Climatology in the Intermountain Region Simulated by the NARCCAP Regional Climate

The central lntermountain Region is a transition zone made up of different climate regimes. ln terms of seasonal cycles of precipitation across this area, different annual and semiannual cycles form a complicated combination of precipitation regimes that meet near Utah. ln terms of climate variability, the central lntermountain Region is situated in a marginal zone of direct influences of El Nino-Southern Oscillation (ENSO). These factors altogether make the climate of the central lntermountain Region particularly difficult to analyze and forecast. While global climate models do not simulate the lntermountain climate very well, a promising alternative are sophisticated regional climate models (RCMs) which tend to show a better representation of the terrain-flow interaction. ln this study we evaluate the precipitation climatology of the lntermountain Region generated by six RCMs of the North American Regional Climate Change Assessment Program (NARCCAP). Although the RCMs produce an overall realistic precipitation climatology in the lntermountain Region, systematic biases are found specifically over the central part of this area. The simulated annual cycles are universally too strong and the amount of winter precipitation is too large. On the other hand, the semiannual cycles are relatively well produced. However, the strong annual cycles and the excess winter precipitation obscure the signals of spring/summer precipitation. These factors lead to false signals of ENSO in areas particularly on the west sides of both the Wasatch Range and the Colorado Rockies. Similar deficiencies are also found in the future scenario of the RCM simulations. Despite the listed model biases, the NARCCAP RCMs do show marked improvement toward a well-handled terrain-flow interaction and accurate phases of seasonal variations. Strong correlations between the RCM precipitation and the Great Salt Lake level fluctuation are found. This indicates a reasonable terrestrial hydrological cycle and highlights the potential on climate impact assessments. Precipitation in the central lntermountain Region is characterized by a pronounced quasi-decadal signal of which the RCMs are capable of capturing. The success in simulating the quasi-decadal variability is particularly encouraging because it enhances the possibilities of long-range climate predictions