Abstract. Changes in cloud condensation nuclei (CCN) concentration induced by anthropogenic aerosols can alter the physical properties of clouds with possible impact on the radiative balance. While strong evidence of local effects comes from studies of clouds induced by ship tracks and by aircraft contrails, the effect of anthropogenic aerosol on clouds at regional and global scales is mainly estimated using numerical models. One issue related to these estimations is the uncertainty in the assumed relationship CCN-sulfate concentration. This study explores the relationship CCN-sulfate using the high-resolution limited area non-hydrostatic model developed at the Geophysical Fluid Dynamics Laboratory. The model has an explicit cloud microphysics scheme and a representation of aerosols consisting of sulfates and sea-salt. The activation of sulfate and sea-salt aerosols is treated explicitly as a function of local model-predicted variables and the simulations presented are representative for remote marine environment conditions. The dependence of CCN on the available sulfate aerosol, NH3, convection intensity, horizontal wind intensity, and the parameters of the particle size distribution are illustrated and compared with observations made during the Atlantic Stratocumulus Transition Experiment/Marine and Gas Exchange experiment. We found that the simulated CCN-sulfate relationship is in good agreement with available observations for a large range of conditions and that significant dispersion in this relation can be attributed to variability in updraft intensity, horizontal wind and chemical composition. This study suggests that cloud resolving models can account for cloud droplet activation directly from aerosol size-distribution information and from predicted thermodynamic local variables.
Copyright 1999 by the American Geophysical Union.