Mesoscale Convective Systems in Weakly Forced Large-Scale Environments. Part III: Numerical Simulations and Implications for Operational Forecasting

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of mesoscale convective systems developed within a weakly forced large-scale environment. Two of these systems had a large component of motion against the midtropospheric flow and propagated in a direction nearly opposite to that of the travelling upper-level disturbances. This evolution of convection is very different from traditional ones in which convection develops and moves more or less in phase with traveling upper-level disturbances. It presents a tremendous challenge for three-dimensional numerical models, since the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conventional upper-air network and may not be well approximated in the model parameterization schemes. Mesoscale model simulations are conducted to evaluate the ability of the model to reproduce this complex event and to examine the model sensitivities to differences in the convective trigger function and model initial condition. Results suggest deal mesoscale models may be capable of producing useful simulations of convective events associated with weak, large-scale forcing, including quantitative precipitation forecasts with the correct magnitude and approximate location of heavy rainfall. If the important mesoscale circulations are incorporated into the model initial condition and a sufficiently realistic trigger function is used. However, model sensitivities to both the initial condition and the convective trigger function are large. Results indicate that the effects of boundary layer forcing must be included in the trigger function in order to initiate convection at the proper time and location. Timing errors in the initial development of convection of greater than 4 h occur if an unpresentative trigger function is used. Mesoscale features in the model initial condition also play an important role in the development and evolution of convection. The locations of heavy rainfall are shifted by greater than 100 km, or disappear altogether, if particular mesoscale features are not included subjectively in the initial condition. These sensitivities suggest that using an ensemble forecast approach to mesoscale model output needs to be considered seriously as mesoscale models move into the operational forecasting environment.