Multiscale Convective Overturning in Mesoscale Convective Systems: Reconciling Observations, Simulations, and Theory

Abstract

An analysis of how parameterized convection interacts with hydrostatic, explicitly resolved precipitation processes to represent multiscale convective overturning in a mesoscale-resolution numerical simulation is presented. Critically important ingredients of the successful simulation are identified and the degree to which simulations are consistent with observations and theoretical considerations is examined. Of particular concern is how convective parameterization routines reconcile the deep moist absolutely unstable structures that form in mesoscale convective systems. It is found that these structures arise primarily from resolvable-scale vertical motion and that the model responds to these structures not only by maintaining parameterized convection, but also by developing a hydrostatic manifestation of convective overturning on its smallest resolvable horizontal scales. The strongest and most distinctive mesoscale perturbations develop in regions where the mesoscale contribution to convective overturning rivals, and often exceeds, the parameterized contribution. Because the internal features of mesoscale convective systems are poorly resolved by meso-?-scale grid elements in this simulation, their scale tends to be overestimated. However, the model results and observations suggest that models must account for multiscale convective overturning in order to properly characterize convective mass transports. Therefore, it is argued that the manner of representation of the convective process, wherein deep convection is allowed to occur partly as a parameterized subgrid-scale process and partly as a hydrostatic manifestation of convective overturning, is likely to give the optimal numerical solution for modeling systems with meso-?-scale resolution.