CLOUD FACTOR AND SEASONALITY OF THE INDIRECT EFFECT OF ANTHROPOGENIC SULFATE AEROSOLS

A method for evaluation of the indirect shortwave effect of anthropoge nic sulfate aerosols based on the empirically derived cloud climatolog y is described and applied. The method is simple and computationally e fficient which allows many sensitivity tests to be conducted. The test s 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 fr om general circulation model simulations in previous studies. The hemi spheric forcing has a strong seasonal cycle with the northern hemisphe re (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% fro m 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 strat iform cloud layers reduce the indirect forcing by more than a factor o f 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 transmittanc e 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 va riability of the aerosol indirect forcing.