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

Clouds constitute a large uncertainty in global climate modeling and climate change projections as many clouds are smaller than the size of a model grid box. Some processes, such as the rates of rain and snow formation that have a large impact on climate, cannot be observed. The uncertain parameters in the representation of these processes are therefore adjusted in order to achieve radiation balance. Here we systematically investigate the impact of key tunable parameters within the convective and stratiform cloud schemes and of the ice cloud optical properties on the present-day climate in terms of clouds, radiation and precipitation. The total anthropogenic aerosol effect between pre-industrial and present-day times amounts to -1.00 W m(-2) obtained as an average over all simulations as compared to -1.02 W m(-2) from those simulations where the global annual mean top-of-the atmosphere radiation balance is within -1.02 W m(-2). Thus tuning of the present-day climate does not seem to have an influence on the total anthropogenic aerosol effect. The parametric uncertainty regarding the above mentioned cloud parameters has an uncertainty range of 25% between the minimum and maximum value when taking all simulations into account. It is reduced to 11% when only the simulations with a balanced top-of-the atmosphere radiation are considered.