Droplet activation and mixing in large-eddy simulation of a shallow cumulus field

This paper presents the application of a double-moment bulk warm-rain microphysics scheme to the simulation of a field of shallow convective clouds based on Barbados Oceanographic and Meteorological Experiment (BOMEX) observations. The scheme predicts the supersaturation field and allows secondary in-cloud activation of cloud droplets above the cloud base. Pristine and polluted cloud condensation nuclei (CCN) environments, as well as opposing subgrid-scale mixing scenarios, are contrasted. Numerical simulations show that about 40% of cloud droplets originate from CCN activated above the cloud base. Significant in-cloud activation leads to the mean cloud droplet concentration that is approximately constant with height, in agreement with aircraft observations. The in-cloud activation affects the spatial distribution of the effective radius and the mean albedo of the cloud field. Differences between pristine and polluted conditions are consistent with the authors’ previous study, but the impact of the subgrid-scale mixing is significantly reduced. Possible explanations of the latter involve physical and numerical aspects. The physical aspects include (i) the counteracting impacts of the subgrid-scale mixing and in-cloud activation and (ii) the mean characteristics of the environmental cloud-free air entrained into a cloud. A simple analysis suggests that the entrained cloud-free air is on average close to saturation, which leads to a small difference between various mixing scenarios. The numerical aspect concerns the relatively small role of the parameterized subgrid-scale mixing when compared to mixing and evaporation due to numerical diffusion. Although the results are consistent with aircraft observations, limitations of the numerical model due to low spatial resolution call for higher-resolution simulations where entrainment processes are resolved rather than mostly parameterized as in the current study.