Impact of parametric uncertainties on the present-day climate and on the anthropogenic aerosol effect

Abstract

Clouds constitute a large uncertainty in global climate modeling and climate changeprojections as many clouds are smaller than the size of a model grid box. Some pro-cesses, such as the rates of rain and snow formation that have a large impact onclimate, cannot be observed. These processes are thus used as tuning parameters in order to achieve radiation balance. Here we systematically investigate the impactof various tunable parameters within the convective and stratiform cloud schemes andof the ice cloud optical properties on the present-day climate in terms of clouds, radi-ation and precipitation. The total anthropogenic aerosol effect between pre-industrialand present-day times amounts to−1.00 Wm−2obtained as an average over all simulations as compared to−1.02 Wm−2from those simulations where the global annualmean top-of-the atmosphere radiation balance is within±1 Wm−2. The parametric un-certainty when taking all simulations into account has an uncertainty range of 25%between the minimum and maximum value. It is reduced to 11% when only the simu-lations with a balanced top-of-the atmosphere radiation are considered.