The influence of the sub-cloud moisture field on cloud size distributions and the consequences for entrainment

In this study we determine cloud size distributions in Large Eddy Simulations of shallow cumulus clouds. First we determine the effect of fluctuations in the sub-cloud moisture field on the lateral size of clouds. To this end we apply at each time-step a spectral filter to the sub-cloud moisture field in order to prevent the formation of fluctuations with spatial scales that are significantly larger than the depth of the sub-cloud layer. The resulting cloud size distributions are compared to the unfiltered situation. Next, we focus on the impact on the cloud dynamics and on the depth of the cloud layer. In particular, we conduct a study of the lateral entrainment rate and analyse in detail its relation with the lateral size of clouds. The relevance of this study with respect to shallow cumulus parametrisation, as well as to the triggering of deep convection, is discussed.