SIMULATING FLASH-FLOOD EVENTS - IMPORTANCE OF THE SUBGRID REPRESENTATION OF CONVECTION

The Pennsylvania State University-NCAR Mesoscale Model is used to simu late six flash flood events that occurred in the central and eastern U nited States. Results suggest that standard model simulations of 24-h accumulated precipitation provide little indication of the potential f or Rash flood-producing rainfall for most events. Often the model gene rates convective outflow boundaries that rapidly move away from their source region and initiate new convection well removed from the origin al convective activity. This self-sustaining propagation mechanism pre vents long-lived heavy rainfall over a particular region. Three modifi cations to the Kain-Fritsch convective parameterization scheme, each d esigned to test model sensitivity to the convective scheme formulation , are discussed. The modifications include maximizing the convective s cheme precipitation efficiency and altering the scheme's treatment of convective downdrafts. Model results using the three modifications of the convective scheme each show improvements in 24-h precipitation tot als compared to simulations generated using the unmodified convective scheme, Precipitation totals appear to be especially sensitive to the way downdrafts are treated within the convective scheme. In addition, the impact of including the mesoscale details of a previously generate d outflow boundary into the model initial conditions is demonstrated f or one of the flash hood cases. It is shown that including the surface mesoscale details can have a substantial impact upon the magnitude an d location of model precipitation maxima.