A mass flux scheme view of a high-resolution simulation of a transition from shallow to deep cumulus convection

Zhiming Kuang and Chris Bretherton

Journal of Atmospheric Sciences, submitted, 04/05, accepted 11/05


We describe an idealized, high-resolution simulation of a gradually forced transition from shallow, non-precipitating to deep, precipitating cumulus convection, explore how the cloud and transport statistics evolve as the convection deepens, and use the collected statistics to evaluate assumptions in current cumulus schemes. The statistical analysis methodologies that we use do not require tracing the history of individual clouds or air parcels, instead relying on probing the ensemble characteristics of cumulus convection in our large model data set. They appear to be an attractive way for analyzing outputs from cloud-resolving numerical experiments. Throughout the simulation, we find that a) the initial thermodynamic properties of the updrafts at the cloud base have rather tight distributions; b) contrary to the assumption made in many cumulus schemes, nearly undiluted air parcels are too infrequent to be relevant to any stage of the simulated convection; and c) a simple model with a spectrum of entraining plumes appears to reproduce most features of the cloudy updrafts, but significantly over-predicts the mass flux as the updrafts approach their levels of zero buoyancy. A buoyancy-sorting model was suggested as a potential remedy. The organized circulations of cold pools seem to create clouds with larger-sized bases and may correspondingly contribute to their smaller lateral entrainment rates. Our results do not support a mass flux closure based solely on CAPE, and are in general agreement with a CIN-based closure. The general similarity in the ensemble characteristics of shallow and deep convection and the continuous evolution of the thermodynamic structure during the transition provide justification for developing a single unified cumulus parameterization that encompasses both shallow and deep convection.
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Zhiming Kuang <kuang@fas.harvard.edu>