A numerical model of the cloud-topped planetary boundary-layer: Impact of aerosol particles on the radiative forcing of stratiform clouds

In a numerical sensitivity study, the microphysical model of stratus MISTRA is used to investigate the impact of aerosol particles on the evolution of stratiform clouds. Four model runs are presented, each for a different type of background aerosol. Two include aerosol particle size-distributions which are typical of marine and rural continental air masses; a third represents a mixture of marine and rural continental aerosol particles, and the fourth rural continental aerosol particles with a reduced solubility in water. The results show that the microphysical structure of layer clouds was strongly affected by the physicochemical properties of the aerosol particles from which the cloud droplets grew. The relatively low concentration of the typical marine aerosol resulted in clouds with a low concentration of large droplets. By contrast, the rural continental and mixed aerosols yielded higher concentrations of relatively small cloud-droplets. Because of their low solubility in water, relatively few of the aerosol particles simulated in the fourth run of the model became activated, so that the microstructure of the cloud was again similar to that with marine aerosol. The different size-distributions of the cloud droplets had a direct influence on the radiative forcing of the clouds. When there were many small droplets, the reflectivity of the clouds was distinctly higher than when there were fewer, but larger, cloud-droplets. In all simulations, the effective radii of the cloud droplets showed a strong diurnal variation.