Comment on wcd-2022-17

Abstract

The relative impact of individual and combined uncertainties of cloud condensation nuclei (CCN) concentration and the shape parameter of the cloud drop size distribution (CDSD) in the presence of initial and boundary condition uncertainty (IBC) on convection forecasts is quantified using the operational convection-permitting model ICON-D2. We performed 180-member ensemble simulations for five real case studies representing different synoptic forcing situations over Germany and inspect the precipitation variability on different spatial and temporal scales. During weak synoptic control, the relative impact of combined microphysical perturbations on area-averaged daily precipitation comprises about ±12 % which is around one-third the variability caused by operational IBC perturbations. The combined microphysical perturbations exceed the impact of individual CCN or CDSD perturbations. High CCN concentrations combined with a narrow CDSD show the largest decrease in precipitation. The combination of IBC and microphysical perturbations affect the extremes of daily spatially averaged rainfall of individual members by extending the tails of the forecast distribution by 5 % in weakly forced conditions. The responses are relatively insensitive in strong forcing situations. Visual inspection and objective analysis of the spatial variability of hourly rainfall rates reveal that IBC and microphysical perturbations alter the spatial variability of precipitation forecasts differently. Microphysical perturbations slightly shift convective cells but affect precipitation intensities while IBC perturbations scramble the location of convection during weak control. Cloud and rain water content is more sensitive to microphysical perturbations than precipitation but slightly less dependent on the synoptic control. In contrast to the impact on precipitation, an increase in CCN concentration and shape parameter of CDSD has a significant positive impact on the formation of cloud water. Combined microphysical perturbations play a dominant role in cloud forecasts with a relative impact ranging between +79 % and -62 % on daily averaged vertically integrated cloud water, and between +57 % and -35 % on rain water content in weakly forced conditions. Thus microphysical uncertainty exhibits a relevant impact on cloud and rain water content and precipitation and its impact largely depends on the prevailing synoptic control in mid-latitude warm-season weather forecasts.