ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-6-947-2006 Constraining the total aerosol indirect effect in the LMDZ and ECHAM4 GCMs using MODIS satellite data Quaas J. 1 3 Boucher O. 2 3 Lohmann U. 4 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 27 03 2006 6 4 947 955 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/6/947/2006/acp-6-947-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/947/2006/acp-6-947-2006.pdf

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 &minus;0.5 and &minus;0.3 Wm<sup>&minus;2</sup>, for LMDZ and ECHAM4, respectively.