Effects of quasi-stationary large-scale anomalies on some mesoscale features associated with the 1993 flood: A regional model simulation

We investigate the effect of quasi-stationary large-scale anomalies on meso scale factors contributing to the 1993 U.S. Midwest flood by performing 30- day simulations with a mesoscale model. The contributions of anomalous meso scale features to the flood were quantified by contrasting model integratio ns forced by initial and lateral boundary conditions with and without large -scale anomalous stationary waves that are believed to have been responsibl e for the flood. Supplementing previous investigations, the present study f inds the following: (1) The large-scale anomalies produced environmental co nditions that favored a generation of MCC-like systems rather than individu al convective cells or squall lines that would have produced less precipita tion over large areas. Ambient conditions favoring mesoscale convective com plex (MCC) include an intensified east-west orientation of quasistationary fronts, weaker upper tropospheric inertial stability, and a strong low-leve l jet (LLJ). (2) The anomalous ageostrophic wind component to the far south of the flood region (Texas area) and geostrophic wind component over the f lood region bridged together to form an extended moisture conveyer that sus tained the long-lasting 1993 flood.