A regional climate model (RCM) is used to simulate precipitation variations during 1982-1988 and 1998-2000 over North America. The RCM is based on the MM5 with several improvements that are important for climate applications. The model uses a 30-km horizontal grid spacing and 23 vertical layers below 100 hPa. The time-varying lateral boundary conditions are constructed from the NCEP/NCAR reanalysis and, hence, the simulation is designed to study the downscaling skill of the RCM.

A comparison with rain gauge measurements shows that the RCM realistically reproduces the observed daily variations of precipitation over most of North America. The downscaling skill is extraordinarily high over the northeast U.S. throughout the year and very high in the remaining U.S. except during summer. The RCM, however, fails to produce the North American summer monsoon rainfall, though giving a realistic simulation over the region for the rest of the year. Sensitivity experiments and detailed diagnoses are conducted to understand the RCM downscaling skill and, in general, the underlying physical mechanisms that explain the precipitation variability.