A REGIONAL MODEL STUDY OF THE IMPORTANCE OF LOCAL VERSUS REMOTE CONTROLS OF THE 1988 DROUGHT AND THE 1993 FLOOD OVER THE CENTRAL UNITED-STATES

Regional model experiments for the drought period of May-June-July (MJ J) 1988 and the flood period of MJJ 1993 over the Central Plains of th e United States are conducted to study the contribution of local versu s nonlocal processes to the maintenance and/or enhancement of the cond itions. It is found that the effect of local recycling of evaporated w ater is not important for the overall development of these two extreme climatic regimes as compared to the effect of large-scale moisture fl uxes and synoptic cyclonic activity. In fact, sensitivity experiments indicate that in the Upper Mississippi Basin (UMB) the main effect of decreased evaporation associated with dry soil conditions at the begin ning of the simulated periods is to increase buoyancy, dynamically sus tain convection, and increase precipitation, thereby providing a negat ive feedback mechanism for the drought-flood conditions. Overall, the model shows a reasonably good performance in simulating various charac teristics of surface climatology over the region during these two extr eme periods, with total simulated precipitation being close to observe d in MJJ 1988 and lower than observed by about 25% in MJJ 1993. The si mulated surface hydrologic budgets and different precipitation statist ics over the UMB are also analyzed.