Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 4 2006 10.5194/acp-6-947-2006 http://www.atmos-chem-phys.net/6/947/2006/ http://www.atmos-chem-phys.net/6/947/2006/acp-6-947-2006.html http://www.atmos-chem-phys.net/6/947/2006/acp-6-947-2006.pdf 947 955 2006-03-27 Constraining the total aerosol indirect effect in the LMDZ and ECHAM4 GCMs using MODIS satellite data J. Quaas O. Boucher U. Lohmann Max Planck Institute for Meteorology, Hamburg, Germany Laboratoire d’Optique Atmosphérique, CNRS/Université des Sciences et Technologies de Lille, Villeneuve d’Ascq, France Hadley Centre, Met Office, Exeter, UK Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland Aerosol indirect effects are considered to be the most uncertain yet important anthropogenic forcing of climate change. The goal of the present study is to reduce this uncertainty by constraining two different general circulation models (LMDZ and ECHAM4) with satellite data. We build a statistical relationship between cloud droplet number concentration and the optical depth of the fine aerosol mode as a measure of the aerosol indirect effect using MODerate Resolution Imaging Spectroradiometer (MODIS) satellite data, and constrain the model parameterizations to match this relationship. We include here "empirical" formulations for the cloud albedo effect as well as parameterizations of the cloud lifetime effect. When fitting the model parameterizations to the satellite data, consistently in both models, the radiative forcing by the combined aerosol indirect effect is reduced considerably, down to −0.5 and −0.3 Wm^−2, for LMDZ and ECHAM4, respectively.