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 mesoscale factors contributing to the 1993 U.S. Midwest flood by performing 30-day simulations with a mesoscale model. The contributions of anomalous mesoscale features to the flood were quantified by contrasting model integrations forced by initial and lateral boundary conditions with and without large-scale anomalous stationary waves that are believed to have been responsible for the flood. Supplementing previous investigations, the present study finds the following: (1) The large-scale anomalies produced environmental conditions that favored a generation of MCC-like systems rather than individual convective cells or squall lines that would have produced less precipitation over large areas. Ambient conditions favoring mesoscale convective complex (MCC) include an intensified east-west orientation of quasi-stationary fronts, weaker upper tropospheric inertial stability, and a strong low-level jet (LLJ). (2) The anomalous ageostrophic wind component to the far south of the flood region (Texas area) and geostrophic wind component over the flood region bridged together to form an extended moisture conveyer that sustained the long-lasting 1993 flood.