This study applies the Regional Eta/SSiB model for the North American regional climate study. We mainly focus on the model simulations of the precipitation and other relevant hydro-meteorological variables. It is important to understand its predictability and the dominant factor(s) and parameterization(s), which may affect its climate predictability. To achieve these goals, a series of sensitivity studies has been designed to explore the role of variety of factors in water cycle simulations. These factors include domain size, horizontal resolution, different lateral boundary conditions (NCEP reanalysis, ECMWF reanalysis, NOAA GCIP reanalysis, GCM outputs), sea surface temperature (SST), and convective parameterizations. The summer of 1998 is selected for the experiments.

The results show that the domain size, and lateral boundary conditions have the crucial impact on the simulations. In the domain size experiment, a set of domains has been selected for testing. The domain that covers only continental U.S. produces the best simulations in the temporal evolution and the spatial distribution of the precipitation. When the domain is expanded to the oceans, although the model is still able to produce reasonable large-scale circulation, the simulation of the intensity and the spatial distribution of the precipitation deteriorate dramatically when the ocean areas become large. It is likely that there is a southern barrier for the Eta model with reanalysis data or perhaps there is a critical spatial scale beyond which regional climate starts interacting strongly with the global climate. The 200mb zonal wind and low level moisture transfer play crucial role in proper simulations of the 1998 US summer precipitation. The role of other factors in the regional climate study will also be presented.