Simulating convective events using a high‐resolution mesoscale model

Abstract

Abstract. Four multiscale numerical simulations of convective events are analyzed to determine the essential characteristics of a numerical model which lead to useful simulations of convective events. Although several universities and weather forecasting centers are currently running high-resolution forecast models, the predictability of convective events, especially in the warm season, is still an issue among researchers and forecasters in the meteorological community. This study shows that explicit simulations of convection depend on the high spatial resolution of physiography (particularly topography and top soil moisture), efficient communication between grids of different scales, and initialization procedures that incorporate mesoscale storm features. The purpose here is to contribute to the debate on predictability of convective events through the discussion of four simulations of midlatitude warm season convective events using a multiscale model. For each case, the most important forcing mechanisms that determined the timing and location of the events were identified, and the performance of the simulation in capturing the forcing mechanisms was analyzed. As a first approach to the problem, a subjective analysis of the forcing mechanisms was performed through the inspection of the observations and model fields at the time of convective development. Although we recognize that this case study approach may not provide general results, we believe that it can offer useful insight into the problem.These simulations were originally designed to study the dynamics of the specific convective events and not to forecast the events in real time. ]. The primary difference between the real-time efforts cited above and the approach used here is that we used a two-way-nested grid system. Single grids driven by previously computed forecast lateral boundaries were used in the other setups. Also, in the case of the CAPS setup, the lateral boundaries were generated by another model. Two advantages of the setup used here are the physical and numerical compatibility between the inner grid and its boundaries and the frequency with which the boundaries are updated (every time step). The main disadvantage is, of course, an increase in computer time. RAMS supports any number of two-way interactive nested grids. Since all grids except the coarsest can move to follow a meteorological feature, much memory and computer time can be saved for large mesoscale grids. Although each simulation used a different grid configuration, all simulations were multiscale in the sense that a large coarse grid (of the order of 80-km grid spacing) was used to capture synoptic scale features such as fronts and troughs, and nesting was done to allow explicit representation of moist convection in a subdomain. In some instances (as shown in Table 1), nested grids were initialized later in the simulation to save computer time. The timing to introduce a new grid was subjectively determined by inspection of the next coarsest grid for signs that convection was ...

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