ON ADEQUATE RESOLUTION FOR THE SIMULATION OF DEEP MOIST CONVECTION : THEORY AND PRELIMINARY SIMULATIONS

The proliferation of inexpensive highperformance computer workstations, massivelyparallel architecture, and distributed-memory codes is accelerating interest in cloud-scale modeling. It is also creating an opportunity to explore the model configuration requirements for accurately simulating moist convective processes. Presently, 1 km horizontal grid spacing is commonly applied for convective cloud simulations. With this resolution, the thunderstorm and its main features (updraft, downdraft, etc.) are crudely represented. However, the commonly observed subcloud-scale rotating eddies (about 1 km in scale) that comprise much of the makeup of convective clouds are not accounted for. It is not clear if these structures affect the fidelity of the overall storm simulation. Before examining this issue further, it is noteworthy that a number of recent studies are already using resolutions better than 1 km (e.g., Finley et al. 2000, McCaul and Cohen 2000, Niino and Noda 2000). Much of the motivation for the shift toward higher resolutions stems from an interest in understanding various components of convective storms, i.e., understanding sub-cloudscale features and processes. For example, Adlerman and Droegemeier (2000) were interested in understanding the relationship between the cyclic mesocyclone and cyclic tornado scales. To explore such an issue, it was necessary to resolve shallow, low-level, sub-cloudscale vortices that formed and dissipated on time scales on the order of a few minutes. By increasing the horizontal resolution to approximately 100 m, Adlerman and Droegemeier found that the general structure of the storm remained similar to coarser-resolution simulations, however, there were significant differences in the size of certain sub-cloud-scale features and that, overall, the process appeared to be accelerated. The differences in the simulations occurred in a relatively short period of time (≈ 2 h) and lead one to wonder how the cumulative effect of such differences, when occurring over time periods of 6 h or more and distances of 100 km or more, would change the structure, dynamics and evolution of mesoscale convective systems? Could it be that the size and amplitude of features such as mesohighs, mesolows, mid-level vortices and rear-inflow jets would differ significantly? Would the phase speed, total precipitation and the distribution of precipitation of convective systems be altered? Recent studies such as Adlerman and Droegemeier (2000) suggest that the concerns raised here are becoming an important issue to an increasing fraction of the numerical modeling community. We believe this is for good reason. Consider the following analogy: It is possible to resolve mesoscale convective systems using 20 km grid spacing but, unless the effects of subgridscale moist convective updrafts and downdrafts are included (parameterized), it is not possible to simulate mesoscale convective systems. For thunderstorms, the analogous statement would be: It is possible to resolve individual thunderstorms with 1 km grid spacing but, unless the effects of subgrid-scale turbulent processes are included, it may not be possible to simulate thunderstorms. Therefore, the issue of whether or not subcloudscale eddies are important has taken on additional relevance and a clear theoretical framework to guide the modeling community in configuring cloud models in a manner which will include turbulent processes is desirable. Section 2 presents a theoretically based argument for specifying the resolution necessary to capture turbulent processes in cloud model simulations. The model configuration for our numerical simulations is detailed in Section 3. Some preliminary results showing how the character of the flow changes as the grid spacing becomes appropriate for large-eddy simulation are presented in Section 4. A brief summary follows in Section 5.