Estimates of aerosol radiative forcing from the MACC re-analysis
1Met Office Hadley Centre, Exeter, UK
2Universität Leipzig, Leipzig, Germany
3European Centre for Medium-range Weather Forecast, Reading, UK
4Laboratoire de Météorologie Dynamique, IPSL, CNRS/UPMC, Paris, France
Abstract. 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 −0.5 Wm−2. Accounting for uncertainties in the aerosol anthropogenic fraction, aerosol absorption, and cloudy-sky effects, results in the direct radiative forcing being bounded by −0.8 and 0 Wm−2. Further accounting for differences between the present-day natural and pre-industrial aerosols provides a direct radiative forcing estimate in the range −0.5 to 0 Wm−2, with a best estimate of −0.3 Wm−2. The best estimate of globally-averaged, all-sky first indirect radiative forcing is −0.4 Wm−2. Accounting for uncertainties in the aerosol anthropogenic fraction, cloud albedo, and cloud droplet number concentration susceptibility to aerosol changes, lower and upper bounds of the first indirect radiative forcing are −2.1 Wm−2 and −0.1 Wm−2 In order to decrease uncertainty ranges, better observational constraints on aerosol absorption and susceptibility of cloud droplet number concentrations to aerosol changes are required.