Systematic Multiscale Models for Deep Convection on Mesoscales

Abstract

This paper builds on recent developments of a unified asymptotic approach to meteorological modelling (Klein (2000), Klein (2003)), which was used successfully in the development of “Systematicmultiscale models for the tropics” in (Majda & Klein (2003), Majda & Biello (2004), Biello & Majda(2005)). Here we account for typical bulk microphysics parameterizations of moist processes within this framework. The key steps are careful nondimensionalization of the bulk microphysics equations and the choice of appropriate distinguished limits for the various nondimensional small parameters that appear. We are then in the position to study scale interactions in the atmosphere involving moist physics. We demonstrate this by developing two systematic multiscale models that are motivated by our interest in mesoscale organized convection. The emphasis here is on multiple length, but common time scales. The first of these models describes the short time evolution of slender, deep convective “hot towers” with horizontal scale 1 km interacting with the linearized momentum balance on length and time scales of (10km / 3 min). We expect this model to describe how convective inhibition may be overcome near the surface, how the onset of deep convection triggers convective scale gravity waves, and that it will also yield new insight into how such local convective events may conspire to create larger scale strong storms. The second model addresses the next larger range of length and time scales (10 km, 100 km, and 20min) and exhibits mathematical features that are strongly reminiscent of mesoscale organized convection. In both cases, the asymptotic analysis reveals how the stiffness of condensation/evaporation processes induces highly nonlinear dynamics. Besides providing new theoretical insights, the derived models may also serve as a theoretical devices for analyzing and interpreting the results of complex moist process model simulations, and they may stimulate the development of new, theoretically grounded subgrid scale parameterizations.