MESOSCALE CONVECTIVE SYSTEMS IN WEAKLY FORCED LARGE-SCALE ENVIRONMENTS .3. NUMERICAL SIMULATIONS AND IMPLICATIONS FOR OPERATIONAL FORECASTING

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of meso scale convective systems developed within a weakly forced large-scale environment. Two of these systems had a large component of motion agai nst the midtropospheric flow and propagated in a direction nearly oppo site to that of the traveling upper-level disturbances. This evolution of convection is very different from traditional ones in which convec tion develops and moves more or less in phase with traveling upper-lev el disturbances. It presents a tremendous challenge for three-dimensio nal numerical models, since the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conv entional upper-air network and may not be well approximated in the mod el 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 convectiv e trigger function and model initial condition. Results suggest that m esoscale models may be capable of producing useful simulations of conv ective events associated with weak, large-scale forcing, including qua ntitative precipitation forecasts with the correct magnitude and appro ximate location of heavy rainfall, if the important mesoscale circulat ions are incorporated into the model initial condition and a sufficien tly realistic trigger function is used. However, model sensitivities t o both the initial condition and the convective trigger function are l arge. Results indicate that the effects of boundary layer forcing must be included in the trigger function in order to initiate convection a t the propel time and location. Timing errors in the initial developme nt of convection of greater than 4 h occur if an unrepresentative trig ger function is used. Mesoscale features in the model initial conditio n also play an important role in the development and evolution of conv ection. The locations of heavy rainfall are shifted by greater than 10 0 km, or disappear altogether, if particular mesoscale features are no t included subjectively in the initial condition. These sensitivities suggest that using an ensemble forecast approach to mesoscale model ou tput needs to be considered seriously as mesoscale models move into th e operational forecasting environment.