Global climate model and coupled regional climate model simulations over the eastern United States: GENESIS and RegCM2 simulations

A Limited Area Model (LAM) is driven at the boundaries by the GENESIS Global Climate Model (GCM) that uses observed sea surface temperatures (SSTs) during December 1979 through June 1980. During this period, observations show that the eastern United States experienced dry conditions in winter, in the spring the Northern Plains experienced drought. Two coupled GCM/LAM sensitivity experiments are undertaken, aimed at attempting to predict these observations while also examining the effects of domain size on the model results. The simulated winter and spring seasons of 1980 from the GENESIS GCM and coupled regional climate are compared to the ECMWF analyses. Our results show significant differences in the large-scale circulation fields (500-mb geopotential heights and 200-mb zonal winds) of GENESIS when compared to observations. The GCM captures the pattern of precipitation but it overestimates precipitation amounts. The coupled simulation which uses a small domain covering the eastern United States (US) and adjacent Atlantic Ocean also overestimates precipitation. However, considerable improvement in precipitation amounts and distribution occur when a continental sized domain is used. The domain captures wetter conditions in the western US during the winter of 1980 and the drier conditions over the Northern Plains during the spring of 1980. Even with the improvements in the simulated precipitation, the boundary conditions from GENESIS produce similar climate statistics in the continental scale regional climate simulations which differ significantly from the ECMWF analyses.