A Budget Analysis of the Variances of Temperature and Moisture in Precipitating Shallow Cumulus Convection

Abstract

Large-eddy simulations of an evolving cloud field are used to investigate the contribution of microphysical processes to the evolution of the variance of total water and liquid water potential temperature in the boundary layer. While the first hours of such simulations show a transient behaviour and have to be analyzed with caution, the final portion of the simulation provides a quasi-equilibrium situation. This allows investigation of the budgets of the variances of total water and liquid water potential temperature and quantification of the contribution of several source and sink terms. Accretion is found to act as a strong sink for the variances, while the contributions from the processes of evaporation and autoconversion are small. A simple parametrization for the sink term connected to accretion is suggested and tested with a different set of simulations.