THE RESOLUTION DEPENDENCE OF EXPLICITLY MODELED CONVECTIVE SYSTEMS

The representation of convective processes within mesoscale models wit h horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the au thors investigate the effects of grid resolution on convective process es using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in me soscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decrease d is documented and the processes that are not properly represented wi th the coarser resolutions are identified. Results from quasi-three-di mensional squall-line simulations for midlatitude-type environments su ggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolut ions is characteristically slower, with the resultant mature mesoscale circulation becoming stronger than those produced in the 1-km case. I t is found that the slower evolution in the coarse-resolution simulati ons is largely a result of the delayed strengthening of the convective cold pool, which is crucial to the evolution of a mature, upshear-til ted convective system The relative success in producing realistic circ ulation patterns at later times for these cases occurs because the col d pool does eventually force the system to grow upscale, allowing it t o be better resolved. The stronger circulation results from an overpre diction of the vertical mass transport produced by the convection at t he leading edge of the system, due to the inability of the coarse-reso lution simulations to properly represent nonhydrostatic effects.