Characteristics and Representation of Subgrid Convective Flux in a Tropical Cyclone Convection System at Convection-Permitting Resolution

Abstract

A large-eddy simulation (LES) of an idealized tropical cyclone convection system was conducted as a benchmark to provide the statistical characteristics of subgrid convective flux at convection-permitting resolution and to evaluate three existing scale-adaptive convection parameterization schemes in the Weather Research and Forecasting model and a nonlinear horizontal gradient (H-gradient) term. Coarse-grained results showed that the vertical profiles of subgrid convective flux presented various modes and each mode exhibited a thin vertical extent, in contrast to the ensemble-mean profile with a bottom-heavy, deep structure. A good spatial correspondence between subgrid convective flux and grid-scale vertical velocity was found in the LES benchmark. The existing convection parameterization schemes were unable to reasonably represent such vertical profiles and the spatial distributions of subgrid convective flux, particularly at high levels. The H-gradient term was able to represent these characteristics of subgrid convective flux, both in terms of vertical profile and spatial distribution. The limitations of traditional mass-flux convection parameterization schemes at convection-permitting resolution and related underlying assumptions are discussed. The physical backgrounds and significances of mass-flux convection parameterization schemes and the H-gradient term are clarified.