We describe the sensitivity of regional simulations of the 1993 summer floods of the Mississippi River Basin to the presence and absence of lateral boundary constraints. The variable resolution Utah global model is used in configurations that allow various degrees of prespecification of the atmosphere external to a limited central region of high resolution, and so permit evaluation of this constraint of limited area models.
Several 13-day simulations are described using initialization obtained from the NCEP/NCAR Reanalysis. The atmospheric state in the outer domain was also partly specified from the Reanalysis. Preliminary results show that the rainfall pattern and its magnitude are sensitive to the degree of prespecification of the outer domain. In particular, pre-specification of the divergent wind in the outer domain has substantial influence upon the rainfall and regional moisture balance simulated within the inner, high-resolution region. In consistency with many other studies, the principal moisture source for the Central U.S. floods appears to be rooted in moisture influx from the Gulf of Mexico via the low-level-jet