The Effects of Domain Choice on Summer Precipitation Simulation and Sensitivity in a Regional Climate Model

Abstract

Recent results show disagreement between global and limited-area models as to the role of soil moisture feedback during the summer of 1993 in the central United States. July precipitation totals increase by 50% in the European Centre for Medium-Range Weather Forecasts global model when soil moisture is initialized ?wet,? but two separate regional modeling groups [University of Utah Limited Area Model group and National Center for Atmospheric Research Regional Climate Model (RegCM) group] have found very different responses to soil moisture, indicating that drier soil moisture conditions might actually lead to increased precipitation via an increase in convective instability and an enhancement of the low-level jet from the Gulf of Mexico. To further evaluate the sensitivity results of RegCM in this context, a new suite of simulations, driven by analyses of observations for May?July of 1988 and 1993 is performed. The model domain is larger than in the previous experiments and the sensitivity of predicted seasonal rainfall to ?wet? and ?dry? initial soil moisture is analyzed. In comparing the new simulations with the earlier results, it is found that the simulation of seasonal precipitation as well as its sensitivity to initial soil moisture are affected by domain size and location of the lateral boundaries in both the 1988 and 1993 experiments. The smaller domain captures observed precipitation better in the upper Mississippi basin; however, the sensitivity of precipitation to initial soil moisture appears to be more realistic in the larger domain. While the lateral boundary forcing in the small domain experiments constrains the model to a better overall simulation, it also yields an unrealistic response to internal forcings, which are not consistent with the applied large-scale forcing. These results demonstrate that the domain of a regional climate model must be carefully selected for its specific application. In particular, domains much larger than the area of interest appear to be needed for studies of sensitivity to internal forcings.