Sensitivity study of cloud parameterizations with relative dispersion in CAM5.1: model evaluation and impacts on aerosol indirect effects

Aerosol-induced increase of relative dispersion of cloud droplet size distribution ε exerts a warming effect and partlyoffsets the cooling of aerosol indirect radiative forcing (AIF) associated with increased droplet concentration by increasing thecloud droplet effective radius (Re) and enhancing the cloud-to-rain autoconversion rate (Au) (labeled as dispersion effect),which can help reconcile global climate models (GCMs) with the satellite observations. However, the total dispersion effectson both Re and Au are not fully considered in most GCMs, especially in different versions of the Community Atmospheric5 Model (CAM). In order to accurately evaluate the dispersion effect on AIF, the new complete cloud parameterizations of ReandAu explicitly accounting for ε are implemented into the CAM version 5.1 (CAM5.1), and a suite of sensitivity experimentsis conducted with different representations of ε reported in literature. It is shown that the shortwave cloud radiative forcingis much better simulated with the new cloud parameterizations as compared to the standard scheme in CAM5.1, whereas theinfluences on longwave cloud radiative forcing and surface precipitation are minimal. Additionally, consideration of dispersion10 effect can significantly reduce the changes induced by anthropogenic aerosols in the cloud top effective radius and the liquidwater path, especially in Northern Hemisphere. The corresponding AIF with dispersion effect considered can also be reducedsubstantially, by a range of 0.10 to 0.21 W m−2 at global scale, and by a much bigger margin of 0.25 to 0.39 W m−2 for theNorthern Hemisphere in comparison with that fixed relative dispersion, mainly dependent on the change of relative dispersionand droplet concentrations (∆ε/∆Nc).15