Evaluation of the ''Twomey effect'' with numerical cloud models and comparisons with observations''

Assessments of the effects of increased aerosol concentrations on cloud albedo, and hence on the global radiative hear budget, are highly uncertain. Estimates and measurements of the sensitivity of cloud albedo to local changes in droplet concentrations indicate that the greatest absolute changes in albedo are found where concentrations of cloud droplets (and aerosols) are lowest. However, determination of effects of pollution sources on the albedo of clouds downwind requires an evaluation of changes in aerosol concentrations during transport from their source regions. Here we use results from a numerical model to calculate the sensitivity of cloud albedo to changes in aerosol concentrations from an upwind source of pollution. We find the greatest increases in cloud albedo to occur close to pollution sources, rather than well downwind in comparatively clean regions.