Abstract. The indirect forcing of climate by atmospheric aerosols is related to the impact of aerosol number concentration on the cloud drop number concentration, which is determined by the cloud condensation nuclei (CCN). For example, an increase of CCN due to pollution can lead to a lower mean drop size and can alter the shortwave albedo of clouds. The estimation of this effect with numerical climate models requires a relationship between the CCN and the aerosol mass concentration. Extensive experimental data suggests a relationship between CCN and the sulfate concentration in cloud drops, as well as a relationship between CCN and the total condensation nuclei concentration below the cloud base. While these relationships provide a simple parameterization of the CCN as a function of sulfate loading, they do not account for local variability caused by model dynamics such as vertical velocity, chemical composition of aerosol, and variations in relative humidity. The current work explores the relationship between CCN and sulfate aerosol loading in marine environment. We use an explicit treatment of cloud microphysics and activation of aerosol particles in the Geophysical Fluid Dynamics Laboratory high-resolution limited area non-hydrostatic model. The results confirm the available empirical analysis within the experimental dispersion, and suggest that significant variability in the relationship CCN-sulfate can be attributed to variations in the intensity of convection, chemical composition of aerosol, and horizontal wind intensity. Implications for the changes in the optical properties of clouds are discussed.
Copyright 1999 by the American Meteorological Society