Examination of atmospheric hydrological processes in RCM simulations

A diagnosis of atmospheric hydrological processes in regional climate models and a test of potential improvements to model representation of such processes are given. The diagnosis consists of an intercomparison of thirteen regional climate models with each other and with observations. The results suggested that the effects of mesoscale convective systems---an integral component of the atmospheric hydrological cycle---are not reliably simulated. A follow-up experiment was conducted in which several configurations of a single regional climate model were examined for improved representation of mesoscale convective systems.; Results from thirteen regional climate model simulations of the 1993 June--July Midwest United States flood were compared to each other and observations. Common data sets were used to generate initial and boundary conditions, and nominal domain size and simulation duration were specified. Each model had unique discretization of dynamic equations and map projection. Furthermore, parameterized models of unresolved processes differed. The results of the intercomparison showed large-scale aspects of the atmospheric hydrological cycle were well simulated in all regional climate models. Nine of the thirteen simulations improved upon the coarser data driving the simulations by more realistically coupling nocturnal precipitation with low-level jets, a process that is completely absent in the driving data. However, some aspects of the coupling mechanism were incorrect, failing to reproduce signals associated with mesoscale convective systems. This incorrect mechanism caused low bias of daily precipitation rate, incorrect location of maximum accumulated precipitation, and incorrect diurnal cycle of precipitation and water vapor convergence.; Test simulations of the 1993 June--July Midwest United States flood were conducted with one widely used regional climate model. The experimental simulations examined separately and in tandem the effects of reducing the horizontal resolution of the regional climate model and modifying its representation of unresolved moist convection. The diurnal cycle of precipitation was most realistic in the tandem experiment. Furthermore, precipitation accumulated over periods consistent with the time scale of mesoscale convective systems was improved by as much as 100%. Future experiments to examine the further potential for improvement were discussed.