Constraining the total aerosol indirect effect in the LMDZ and ECHAM4 GCMs using MODIS satellite data

Abstract

Aerosol indirect effects are considered to be the most uncertain yet important an-thropogenic forcing of climate change. The goal of the present study is to reducethis uncertainty by constraining two different general circulation models (LMDZ andECHAM4) with satellite data. We build a statistical relationship between cloud droplet number concentration and the optical depth of the fine aerosol mode as a measureof the aerosol indirect effect using MODerate Resolution Imaging Spectroradiometer(MODIS) satellite data, and constrain the model parameterizations to match this rela-tionship. We include here “empirical” formulations for the cloud albedo effect as well asparameterizations of the cloud lifetime effect. When fitting the model parameterizations to the satellite data, consistently in both models, the radiative forcing by the combinedaerosol indirect effect is reduced considerably, down to−0.5 and−0.3Wm−2, for LMDZand ECHAM4, respectively.