Nested-Model Simulation of Moist Convection: The Impact of Coarse-Grid Parameterized Convection on Fine-Grid Resolved Convection

Abstract

Future-generation, operational, weather prediction systems will likely include storm-scale, limited-area models that will explicitly resolve convective precipitation. However, the high-resolution convection-resolving grids will need to be embedded, or nested, within coarser-resolution grids that will provide lateral-boundary conditions. It is the purpose of this study to illustrate how the convective environment on a convection-resolving storm-scale model grid, and therefore the convection itself, can be significantly influenced by the treatment of convection on the coarser grids within which the fine grid is embedded. It was confirmed that, as in the actual atmosphere, mass-field adjustments resulting from convection in one area (the outer grids, in this case) affect the convection in other areas (the inner, convection-resolving grid). That is, the errors in precipitation timing, precipitation intensity, and the vertical distribution of latent heating, associated with the treatment of convection on the outer grids, greatly affect the explicit convection on the inner grid. In this case, the different precipitation parameterizations on the outer two grids produce up to a factor of 3 difference in the 24-h amount of explicit rainfall simulated on the inner grid. Even when the parameterization is limited to only the outer grid, with explicit precipitation on both the middle and inner grids, over a factor of 2 difference in 24-h total explicit rainfall is produced on the inner grid. The different precipitation parameterizations on the outer grids appear to differently modulate the intensity and the timing of the explicit convection on the inner grid through induced subsidence.