Numerical simulation of the 1993 midwestern flood: Land-atmosphere interactions

During the summer of 1993, persistent and heavy precipitation caused a long-lived, catastrophic flood in the midwestern United States. In this paper, Midwest hydrology, atmospheric circulation of the 1993 summer, and feedback between the surface and precipitating systems were investigated using the Purdue Regional Model (PRM). The 30-day PRM control simulations reproduced the large-scale atmospheric features that characterized the summer of 1993. Specifically, the upper-level jet stream and trough over the northwestern United States are present in control cases, as well as the Great Plains low-level jet, general pattern of moisture transport, and heavy precipitation in the Midwest. The daily precipitation record (area averaged over the heaviest rainfall) indicates that the model also reproduces the evolution and periodicity of precipitation events comparable with the observations and correctly depicts the differences between June and July. The sensitivity of the low-level jet, planetary boundary layer, and heavy precipitation were examined by imposing various soil moisture and surface anomalies in the model simulation. The increased surface heating, caused by a strong dry anomaly, induced a large scale surface pressure perturbation, centered in the southeastern United States, that weakened the low-level jet and moisture convergence within the flood region. Separate cases considering both wet and dry regional anomalies in the southern Great Plains caused less precipitation in the flood region. The uniform soil moisture of both anomalies leads to a reduction of the differential heating, surface pressure gradient, and the low-level jet.