An investigation of the effect of sulfate on cloud microphysics using a chemistry/transport model

A feasibility study has been made using information from satellite radiances and a sulfate transport model to quantify possible indirect radiative effects of aerosols on cloud properties. AVHRR GAC data for the North Atlantic during April 1987 were used to extract cloud microphysical properties such as liquid water path, column droplet concentration and effective radius. A meteorology driven simplified chemistry model was used to identify regions and time periods during which there was an intrusion of sulfate rich air into the North Atlantic from Europe.

A comparison of cloud microphysical parameters from clean and polluted samples showed that it is possible to interpret changes in effective radius. Questions about changes in optical depth and liquid water path are much more difficult to address for two reasons. The horizontal inhomogeneity of cloud fields results in an underestimate of optical depth and hence, liquid water path, and this bias depends on the degree of variability which need not be the same for different samples. In addition, non-adiabatic effects could also vary from sample to sample. Nevertheless, with these two caveats, a global analysis using satellite data and transport model output can be used to constrain some of the uncertainties in the magnitude of the indirect aerosol radiative effect on the Earth's radiation budget.