Abstract
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 −0.7 ± 0.3 Wm−2. 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−2. The best estimate of globally-averaged, all-sky first indirect radiative forcing is −0.6 ± 0.4 Wm−2. 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−2. In order to decrease uncertainty ranges, better observational constraints on aerosol absorption and sensitivity of cloud droplet number concentrations to aerosol changes are required.
Highlights
The Monitoring Atmospheric Composition and Climate (MACC) total DRE is computed as the sum of the component DREs: this method neglects the coupling between the DRE of different species and overestimates the DRE by a few tenths of Wm−2, the exact value depending on the relative vertical profiles of the different aerosol species
On a multi-annual, global average, the best estimate of IRF is −0.6 Wm−2 (Table 3). This estimate is stronger than the −0.2 Wm−2 derived from satellite data by Quaas et al (2008) because the anthropogenic aerosol optical depths (AODs) is larger in MACC, especially at low latitudes over the oceans where cloud susceptibility is large
Estimation of aerosol forcing from the MACC aerosol re-analysis uses data-assimilated aerosol products from European Centre for Medium-range Weather Forecast (ECMWF) in order to combine the strengths of satellitebased estimates, which are tied to actual aerosol distributions, with free-running model estimates, which do not include data gaps and may provide additional information
Summary
The aerosol analysis and forecast system of the ECMWF IFS consists of a forward model (Morcrette et al, 2009) and a data-assimilation module (Benedetti et al, 2009). Mineral dust and sea salt are represented by three different size classes each, and hydrophilic and hydrophobic modes of BC and OM are distinguished. Data assimilation consists in the minimisation of a complicated cost-function and updates the modelled aerosol masses in order to match observations more closely. Each aerosol component is corrected in proportion of its original contribution to the total aerosol mass This mass increment is converted to an AOD and compared against the assimilated MODIS retrieval, until convergence is achieved within observational and model errors. The guiding principle of the derivation of aerosol forcing from the aerosol distributions of the MACC re-analysis is to rely first on variables that are affected by data assimilation and combine them with observational estimates of other variables, such as aerosol optical properties or cloud susceptibility to aerosol changes. Fine-mode particles are those with radii smaller than 0.5 μm, in agreement with the definition used for the MODIS retrieval of the FMF
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