A budget analysis of variance and skewness of moisture in shallow cumulus convection

For cloud modelling on all scales a good understanding of the spatial variability of moisture is essential. In order to develop and improve scale aware cloud parameterizations for climate or weather prediction models, assumptions about the sub-grid variability of moisture have to be made and evaluated.

The focus of this study is on situations of shallow convection. By using large eddy simulations the budget equations of variance and skewness of total water are analysed. Several tendencies contributing to the time evolution of variance and skewness of total water are evaluated based on the LES data. Besides the effect of turbulent transport and production, a special focus is also on the tendencies due to microphysical processes (evaporation of rain, autoconversion and accretion). These tendencies are investigated with respect to magnitude, sign and structure. The obtained information will be helpful for understanding the effects of the microphysical processes on the cloud dynamics, e.g, through the formation of cold pools due to evaporation of rain in the sub-cloud layer. A detailed analysis of the evolution of higher statistical moments will also help to improve the process-level understanding of possible feedbacks leading to self-organization of clouds and the formation of mesoscale structures.

Eventually, this analysis should lead to improved parameterizations of cloud processes in higher-order closure boundary layer schemes as well as PDF-based cloud schemes.