Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 11 2006 10.5194/acp-6-3583-2006 http://www.atmos-chem-phys.net/6/3583/2006/ http://www.atmos-chem-phys.net/6/3583/2006/acp-6-3583-2006.html http://www.atmos-chem-phys.net/6/3583/2006/acp-6-3583-2006.pdf 3583 3601 2006-09-04 Combined observational and modeling based study of the aerosol indirect effect T. Storelvmo trude.storelvmo@geo.uio.no J. E. Kristjánsson G. Myhre M. Johnsrud F. Stordal Department of Geosciences, University of Oslo, Oslo, Norway Norwegian Institute for Air Research, Kjeller, Norway The indirect effect of aerosols via liquid clouds is investigated by comparing aerosol and cloud characteristics from the Global Climate Model CAM-Oslo to those observed by the MODIS instrument onboard the TERRA and AQUA satellites http://modis.gsfc.nasa.gov). The comparison is carried out for 15 selected regions ranging from remote and clean to densely populated and polluted. For each region, the regression coefficient and correlation coefficient for the following parameters are calculated: Aerosol Optical Depth vs. Liquid Cloud Optical Thickness, Aerosol Optical Depth vs. Liquid Cloud Droplet Effective Radius and Aerosol Optical Depth vs. Cloud Liquid Water Path. Modeled and observed correlation coefficients and regression coefficients are then compared for a 3-year period starting in January 2001. Additionally, global maps for a number of aerosol and cloud parameters crucial for the understanding of the aerosol indirect effect are compared for the same period of time. Significant differences are found between MODIS and CAM-Oslo both in the regional and global comparison. However, both the model and the observations show a positive correlation between Aerosol Optical Depth and Cloud Optical Depth in practically all regions and for all seasons, in agreement with the current understanding of aerosol-cloud interactions. 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