Evolution and maintenance of simulated extreme-rain-producing mesoscale convective systems

When mesoscale convective systems (MCSs) are organized such that new convection develops upstream of the existing convection so that the system as a whole becomes quasi-stationary, the potential often exists for large accumulations of rainfall and flash flooding. One such MCS, occurring on 6-7 May 2000, produced extreme rainfall amounts (in excess of 300 mm in 8 h) and resulted in over $100M in damages from flash flooding in eastern Missouri. This system developed near a mesoscale convective vortex (MCV) in a very moist environment that included a strong low-level jet.

Operational model forecasts and simulations using a convective parameterization scheme failed to produce the observed rainfall totals for this event. However, cloud-resolving simulations using the Weather Research and Forecasting (WRF) model were successful in reproducing the quasi-stationary organization and evolution of this MCS. In both observations and simulations, scattered elevated convective cells were repeatedly initiated 50-75 km upstream before merging into the mature MCS and contributing to the heavy rainfall. The convection was maintained in part by the lifting provided by the MCV, and simulations indicate that the MCS was long-lived despite the lack of a convectively-generated cold pool at the surface.

Additional experiments using an idealized initial state suggest a series of mechanisms for the maintenance and evolution of this type of MCS. These results suggest that there may be potential for increased understanding and, in some cases, improvements in prediction of the convection that produces extreme precipitation and resultant flash flooding.