Intercomparison of Global Reanalyses and Regional Simulations of Cold Season Water Budgets in the Western United States

Abstract

Estimating water budgets of river basins in the western United States is a challenge because of the effects of complex terrain and lack of comprehensive observational datasets. This study aims at comparing different estimates of cold season water budgets of the Columbia River (CRB) and Sacramento?San Joaquin River (SSJ) basins. An intercomparison was performed based on the NCEP?NCAR reanalysis I (NRA1), NCEP?Department of Energy (DOE) reanalysis II (NRA2), ECMWF reanalyses (ERA), regional climate simulations produced by the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) and NCEP Regional Spectral Model (RSM) driven by the reanalyses, and two precipitation datasets gridded at 2.5° and ?° for 7 yr between 1986 and 1993. The purpose of the intercomparison was to understand the effects of spatial resolution, model configuration and associated parameterizations, and large-scale conditions on basin-scale water budgets. Overall, the regional simulations were superior to the global reanalyses in terms of the spatial distribution of mean precipitation and precipitation anomalies. However, cold season precipitation was generally amplified in the regional models. Basin mean precipitation was typically higher than observed in the regional models and less than observed in the reanalyses. The amplification was the largest in the RSM simulation driven by NRA2, which had the biggest difference between the reanalyzed and regional simulation of basin mean precipitation. ERA and the MM5 simulations driven by ERA provided the best basin mean precipitation estimates when compared to the ?° observational dataset. Large differences remain in estimating the water budgets of western river basins, such as CRB and SSJ. In terms of atmospheric moisture flux, there was a 15%?20% difference between the global reanalyses. In terms of basin mean precipitation, differences among the reanalyses, regional simulations, and observations were as large as 100% of the overall mean. There were large differences in spatial distribution of precipitation between the RSM and MM5 simulations because of terrain representations and other factors. Runoff and snowpack showed the most sensitivity to model differences in spatial resolution, physics parameterizations, and model representations. Better simulations of basin mean precipitation did not necessarily imply superior simulations of runoff or snowpack.