Predicting cloud‐droplet effective radius and indirect sulphate aerosol forcing using a general circulation model

Various methods for predicting cloud‐droplet effective radius in the Hadley 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 approaches and of different input sulphate data sets are examined; however, there is no clear evidence to prefer either of the two sulphate data sets used in the study. Two of the parametrizations generate distributions of present‐day effective radius which are similar to each other and compare favourably with observations, yet provide very different estimates 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 that droplet concentrations are not determined by sulphate concentrations 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 understanding of the link between sulphate aerosol mass concentrations, cloud condensation nuclei, and cloud‐droplet number concentrations.