PREDICTING CLOUD-DROPLET EFFECTIVE RADIUS AND INDIRECT SULFATE AEROSOL FORCING USING A GENERAL-CIRCULATION MODEL

Various methods for predicting cloud-droplet effective radius in the H adley Centre general circulation model are compared with aircraft and satellite retrievals, and are used to estimate the indirect radiative forcing by anthropogenic sulphate aerosols since the beginning of the industrial era. The effects both of different parametrization approach es and of different input sulphate data sets are examined; however, th ere is no clear evidence to prefer either of the two sulphate data set s used in the study. Two of the parametrizations generate distribution s of present-day effective radius which are similar to each other and compare favourably with observations, yet provide very different estim ates of the indirect effect, ranging from -0.5 to -1.5 W m(-2) in the global annual mean. A sensitivity experiment in which it is assumed th at droplet concentrations are not determined by sulphate concentration s in continental air reduces this global-mean forcing to -0.3 to -0.8 W m(-2). This sensitivity demonstrates the need for a much better unde rstanding of the link between sulphate aerosol mass concentrations, cl oud condensation nuclei, and cloud-droplet number concentrations.