Combined effects of soil moisture and microphysical perturbations on convective clouds and precipitation for a locally forced case over Central Europe

To study the combined impact of soil moisture and microphysical perturbations on convective clouds and precipitation over Central Europe, an ensemble of five dozen real-world weather prediction forecasts was conducted with the COnsortium for Small-scale MOdeling (COSMO) model at convection-permitting resolution for a case with weak large-scale forcing (6 June 2016). We find a large sensitivity of precipitation, ranging from +10% to − 23% in 12-hr precipitation totals. While the homogeneous soil-moisture bias of ± 25% primarily controls the timing of convection initiation and the amount of surface rainfall, the number of cloud condensation nuclei and width of the cloud droplet size distribution mainly control the number, size, and lifetime of convective clouds. In moisture-limited conditions, mainly positive couplings are acting. Drier soils, cleaner air, and a broader cloud droplet size distribution result in less rainfall. Wetter soils and more polluted conditions lead to fewer, but larger, cloud clusters. Since microphysical process rates depend systematically on the sign of the perturbations, but rainfall does not, there are compensating effects at work that buffer microphysical perturbations directly and impact the cloud condensate amount and the rainfall at the ground.