Spatial-temporal organization of convection and the diurnal variability of precipitation: A U.S. Continent study

As identified in the original prospectus for this special session, there exists complex relationships between the diurnal variability of precipitation and the spatial-temporal organization of deep convection. This behavior has long been known in principle but its global ubiquity has only recently been quantified through the detailed analysis of TRMM datasets. While relationships between spatial-temporal convective organization and diurnal variability also exist over the oceans, they are more complex over continents and coastal regions, notably in association with mountainous orography and in the presence of shear flow. The remote (far-field) effect of mountains in sheared environments is a fundamental aspect. In certain conditions persistent organized convective systems may travel for hundreds and even thousands of kilometers downshear of mountain ranges. This coherent property is poorly represented in contemporary operational global prediction models, even state-of-the-art models. It is entirely absent from contemporary climate models. We investigate the prediction problem over the continental United States using a hierarchica simulations: cloud-system resolving numerical models and models operating convective parameterizations. The U.S.continent is a useful testbed because the national radar network (NEXRAD)provides useful datasets for model verification.

Basic properties such as the propagation, life-cycle, regime of organization, and dynamical transport identifed in the simulations are approximated by a nonlinear dynamical model. Further approximation yields a dynamically based parameterization of mesoscale evaporation/stratiform heating, which is known to be associated with organized traveling convection. This parameterization is simple enough to be readily employed in global weather prediction models and climate models. (Note that mesoscale convective organization is not represented in contemporary convective parameterizations.) This provides a framework for inter-relating the diurnal variability of precipitation and the spatial-temporal organization of convection in global prediction models. The dynamical coupling of the convective to the mesoscale aspects is in the proces of being extended and the ideatested.