A numerical model of the cloud-topped planetary boundary-layer: radiative forcing of aerosols in stratiform clouds

Abstract

In a numerical sensitivity study with the microphysical stratus model MISTRA the impact of aerosol particles on the time evolution of stratiform clouds is investigated. Four model runs with different aerosol size distributions are presented. Two size distributions are typical for maritime and continental air masses. The third model run consists of a mixture of maritime and rural aerosol particles, while in the fourth case study rural aerosol particles with a reduced water solubility are utilized. The numerical results show that the microphysical structure of the clouds is strongly affected by the physico-chemical properties of the aerosol particles. In the maritime case, with a relatively low aerosol concentration, the clouds consist of few but large droplets. In contrast to this, the rural and the mixed case yield many, but relatively small, cloud droplets. Due to the low water solubility of the aerosol particles, in the fourth model run only a small number of aerosol particles becomes activated so that the microstructure of the cloud is again very similar to the maritime case. The different droplet spectra have a direct influence on the radiative forcing of the clouds. In situations with many small droplets the reflectivity of the clouds is distinctly higher than in cases with few but large cloud droplets. The effective cloud droplet radii are larger in the model runs with few large cloud droplets than in the other cases. In all simulations the radii show a strong diurnal variation.