Large‐eddy simulation of the transient and near‐equilibrium behavior of precipitating shallow convection

Journal of Advances in Modeling Earth Systems

Axel Seifert³  Thijs Heus²  Robert Pincus¹ 

[1] University of Colorado, and NOAA/Earth System Research Laboratory, Boulder, Colorado, USA;Department of Physics, Cleveland State University, Cleveland, Ohio, USA;Hans-Ertel Centre for Weather Research, Deutscher Wetterdienst, Hamburg, Germany

关键词: large‐eddy simulation;    cloud‐radiative effects;    shallow convection;    aerosol indirect effects;   

DOI  :  10.1002/2015MS000489

来源: Wiley

【 摘 要 】

Abstract

Large-eddy simulation is used to study the sensitivity of trade wind cumulus clouds to perturbations in cloud droplet number concentrations. We find that the trade wind cumulus system approaches a radiative-convective equilibrium state, modified by net warming and drying from imposed large-scale advective forcing. The system requires several days to reach equilibrium when cooling rates are specified but much less time, and with less sensitivity to cloud droplet number density, when radiation depends realistically on the vertical distribution of water vapor. The transient behavior and the properties of the near-equilibrium cloud field depend on the microphysical state and therefore on the cloud droplet number density, here taken as a proxy for the ambient aerosol. The primary response of the cloud field to changes in the cloud droplet number density is deepening of the cloud layer. This deepening leads to a decrease in relative humidity and a faster evaporation of small clouds and cloud remnants constituting a negative lifetime effect. In the near-equilibrium regime, the decrease in cloud cover compensates much of the Twomey effect, i.e., the brightening of the clouds, and the overall aerosol effect on the albedo of the organized precipitating cumulus cloud field is small.

【 授权许可】

CC BY-NC-ND   
© 2015. The Authors.

Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.