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 M ISTRA is used to investigate the impact of aerosol particles on the ev olution of stratiform clouds. Four model runs are presented, each for a different type of background aerosol. Two include aerosol particle s ize-distributions which are typical of marine and rural continental ai r 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 microph ysical structure of layer clouds was strongly affected by the physicoc hemical properties of the aerosol particles from which the cloud dropl ets grew. The relatively low concentration of the typical marine aeros ol resulted in clouds with a low concentration of large droplets. By c ontrast, the rural continental and mixed aerosols yielded higher conce ntrations of relatively small cloud-droplets. Because of their low sol ubility in water, relatively few of the aerosol particles simulated in the fourth run of the model became activated, so that the microstruct ure of the cloud was again similar to that with marine aerosol. The di fferent size-distributions of the cloud droplets had a direct influenc e on the radiative forcing of the clouds. When there were many small d roplets, the reflectivity of the clouds was distinctly higher than whe n there were fewer, but larger, cloud-droplets. In all simulations, th e effective radii of the cloud droplets showed a strong diurnal variat ion.