ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-13-2045-2013 Estimates of aerosol radiative forcing from the MACC re-analysis Bellouin N. 1 5 Quaas J. 2 Morcrette J.-J. 3 Boucher O. 4 Met Office Hadley Centre, Exeter, UK Universität Leipzig, Leipzig, Germany European Centre for Medium-range Weather Forecast, Reading, UK Laboratoire de Météorologie Dynamique, IPSL, CNRS/UPMC, Paris, France now at: Department of Meteorology, University of Reading, Reading, UK 21 02 2013 13 4 2045 2062 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/13/2045/2013/acp-13-2045-2013.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/2045/2013/acp-13-2045-2013.pdf

The European Centre for Medium-range Weather Forecast (ECMWF) provides an aerosol re-analysis starting from year 2003 for the Monitoring Atmospheric Composition and Climate (MACC) project. The re-analysis assimilates total aerosol optical depth retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) to correct for model departures from observed aerosols. The re-analysis therefore combines satellite retrievals with the full spatial coverage of a numerical model. Re-analysed products are used here to estimate the shortwave direct and first indirect radiative forcing of anthropogenic aerosols over the period 2003–2010, using methods previously applied to satellite retrievals of aerosols and clouds. The best estimate of globally-averaged, all-sky direct radiative forcing is −0.7 ± 0.3 Wm<sup>&minus;2</sup>. The standard deviation is obtained by a Monte-Carlo analysis of uncertainties, which accounts for uncertainties in the aerosol anthropogenic fraction, aerosol absorption, and cloudy-sky effects. Further accounting for differences between the present-day natural and pre-industrial aerosols provides a direct radiative forcing estimate of −0.4 ± 0.3 Wm<sup>&minus;2</sup>. The best estimate of globally-averaged, all-sky first indirect radiative forcing is −0.6 ± 0.4 Wm<sup>&minus;2</sup>. Its standard deviation accounts for uncertainties in the aerosol anthropogenic fraction, and in cloud albedo and cloud droplet number concentration susceptibilities to aerosol changes. The distribution of first indirect radiative forcing is asymmetric and is bounded by −0.1 and −2.0 Wm<sup>−2</sup>. In order to decrease uncertainty ranges, better observational constraints on aerosol absorption and sensitivity of cloud droplet number concentrations to aerosol changes are required.

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