Cloud factor and seasonality of the indirect effect of anthropogenic sulfate aerosols

A method for evaluation of the indirect shortwave effect of anthropogenic sulfate aerosols based on the empirically derived cloud climatology is described and applied. The method is simple and computationally efficient which allows many sensitivity tests to be conducted. The tests presented focus mainly on the effects of cloud amounts and types as well as on seasonality of the indirect forcing. We found the indirect shortwave forcing estimated from cloud climatology (-1.1 W m(-2), with a hemispheric difference of 0.4 W m(-2)) is close to that obtained from general circulation model simulations in previous studies. The hemispheric forcing has a strong seasonal cycle with the northern hemisphere (NH) forcing exceeding that of the southern hemisphere (SH) during the NH spring and summer, and the SH forcing prevailing during the SH spring and summer. We also estimate that single-layer St/Sc contribute most to the aerosol indirect forcing (60%) with an additional 25% from St/Sc overlapped with Ci/Cs/Cc and 15% from St/Sc overlapped with As/Ac. The upper and middle level clouds overlapping the low level stratiform cloud layers reduce the indirect forcing by more than a factor of 2 (from -2.5 to -1.1 W m(-2)). The estimate of the indirect forcing of -1.1 W m(-2) is based on climatological mean values of transmittance for Ac/As and Ci/Cs/Cc. The transmittance of these clouds, however, may vary in a wide range resulting in a large regional and seasonal variability of the aerosol indirect forcing.