Model intercomparison of indirect aerosol effects

Modeled differences in predicted effects are in- creasingly used to help quantify the uncertainty of these ef- fects. Here, we examine modeled differences in the aerosol indirect effect in a series of experiments that help to quan- tify how and why model-predicted aerosol indirect forcing varies between models. The experiments start with an exper- iment in which aerosol concentrations, the parameterization of droplet concentrations and the autoconversion scheme are all specified and end with an experiment that examines the predicted aerosol indirect forcing when only aerosol sources are specified. Although there are large differences in the pre- dicted liquid water path among the models, the predicted aerosol first indirect effect for the first experiment is rather similar, about 0.6 Wm 2 to 0.7 Wm 2 . Changes to the autoconversion scheme can lead to large changes in the liq- uid water path of the models and to the response of the liquid water path to changes in aerosols. Adding an autoconversion scheme that depends on the droplet concentration caused a larger (negative) change in net outgoing shortwave radiation compared to the 1st indirect effect, and the increase varied from only 22% to more than a factor of three. The change in net shortwave forcing in the models due to varying the au- toconversion scheme depends on the liquid water content of the clouds as well as their predicted droplet concentrations, and both increases and decreases in the net shortwave forc- ing can occur when autoconversion schemes are changed. The parameterization of cloud fraction within models is not sensitive to the aerosol concentration, and, therefore, the re- sponse of the modeled cloud fraction within the present mod- els appears to be smaller than that which would be associated with model "noise". The prediction of aerosol concentra- tions, given a fixed set of sources, leads to some of the largest differences in the predicted aerosol indirect radiative forcing among the models, with values of cloud forcing ranging from 0.3 Wm 2 to 1.4 Wm 2 . Thus, this aspect of modeling requires significant improvement in order to improve the pre- diction of aerosol indirect effects.

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