DEPENDENCE OF SIMULATED PRECIPITATION ON SURFACE EVAPORATION DURING THE 1993 UNITED-STATES SUMMER FLOODS

Regional summertime atmospheric conditions of 1993 are analyzed with t he University of Utah Local Area Model (ULAM) by nudging boundary valu es and large internal scales of the local model toward values produced by the Nested Grid Model(NCEP/NOAA) initial analyses and forecasts ar chived at 6-h intervals. The approach allows the local ULAM to develop finer-scale structures in the precipitation and circulation forecasts than those resolved by the NGM. The study focuses on the influence of surface evaporation upon rainfall and low-level flow in regional simu lations. Much of the rainfall simulated in the control experiment occu rred from the late afternoon to early morning hours, with a pronounced midday minimum over the flood region. The moisture flux from the sout h due to the low-level jet (LLJ) provides much of the moisture source for the precipitation, and it is shown that the net moisture influx is significantly larger than the rainfall rate over the flood region. As a consequence, modifications of surface evaporation apparently are re latively more important in changing the buoyancy and resulting LLJ str ength than they are in providing additional moisture to the already pl entiful moisture influx from the Gulf of Mexico. This suggests that ac curate surface evaporation in the Great Plains is necessary for accura te simulation of dynamic support for rainfall. The LLJ and especially its diurnal oscillation increase for drier surface conditions in the v icinity of the jet core, providing more effective convergence patterns to support rainfall in these cases than in cases of stronger surface evaporation. This appears to be a more important mechanism for rainfal l release over the Mississippi River basin than moistening through loc al evapotranspiration, although the latter also contributes to more ra infall when this moistening occurs downwind of the jet core.