The indirect forcing due to anthropogenic aerosols is one of the most uncertain among the possible forcings of climate. This is particularly due to uncertainties in aerosol concentrations and properties, but much of the uncertainty is due to the parameterizations of cloud microphysical processes in climate models. An important component of the cloud microphysical parameterizations is the prediction of cloud droplet number concentration (CDNC) in a physically based manner. The observed wide variations in CDNC can be expected to produce large differences in precipitation efficiency and in the optical properties of clouds. Although such variations in CDNC can be prescribed from observations, there is currently a great interest in predicting CDNC because of the hypothesis that CDNC are influenced by anthropogenic aerosol.

In this study we use the Mesoscale Compressible Community model with the Cohard and Pinty (2000) two-moment warm-phase microphysical scheme and the Abdul-Razzak et al. (1998) nucleation model to simulate at high resolution (2 km) the physical processes that govern the aerosol-cloud interactions in summertime stratus clouds that occurred over the North Atlantic. We compare the model results with in-situ aircraft observations of cloud and aerosol properties. Specifically, we investigate the link between the aerosol spectrum and CDNC. We then examine the sensitivity of modeled cloud microphysical properties (liquid water content and CDNC) to different representations of aerosol. Initially, we prescribe the aerosol number concentration and then allow it to be modified by the cloud microphysical processes and in-cloud oxidation. Finally, we study the response of the stratus cloud albedo and radiative fluxes to variations in ambient aerosol concentrations.