MODEL SIMULATION OF IMPACTS OF TRANSIENT SURFACE WETNESS ON SUMMER RAINFALL IN THE UNITED-STATES MIDWEST DURING DROUGHT AND FLOOD YEARS

Surface moisture availability has been hypothesized by various investi gators to provide additional negative (positive) feedback on rainfall during summer drought (flood) conditions in the Midwest. In this note, we report on a preliminary numerical modeling effort in which the imp act of transient changes in surface wetness on summer rainfall events in the midwestern United States during two recent drought and flood ye ars is assessed. It was found that during the drought of 1988, hypothe tical temporary extreme moistening of the surface resulted in large re lative increases in simulated rainfall, often by as much as a factor o f 2. However, from an agricultural perspective these large relative ch anges in rainfall might not necessarily have translated into meaningfu l increases since the original absolute rainfall amounts were quite sm all. In the flood year of 1993, an assumed transient drying of the sur face resulted in relative decreases in simulated rainfall by as much a s 30%-40%. This relative decrease in rainfall did, however, translate into a discernible drop in the absolute rainfall.