Dependence of Simulated Precipitation on Surface Evaporation during the 1993 United States Summer Floods

Regional summertime atmospheric conditions of 1993 are analyzed with the University of Utah Local Area Model (ULAM) by nudging boundary values and large internal scales of the local model toward values produced by the Nested Grid Model (NCEP/NOAA) initial analyses and forecasts archived 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 simulations. Much of the rainfall simulated in the control experiment occurred from the late afternoon to early morning hours, with a pronounced midday minimum over the flood region. The moisture flux from the south 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 ...