Abstract
Abstract. Using the GlobAEROSOL-AATSR dataset, estimates of the instantaneous, clear-sky, direct aerosol radiative effect and radiative forcing have been produced for the year 2006. Aerosol Robotic Network sun-photometer measurements have been used to characterise the random and systematic error in the GlobAEROSOL product for 22 regions covering the globe. Representative aerosol properties for each region were derived from the results of a wide range of literature sources and, along with the de-biased GlobAEROSOL AODs, were used to drive an offline version of the Met Office unified model radiation scheme. In addition to the mean AOD, best-estimate run of the radiation scheme, a range of additional calculations were done to propagate uncertainty estimates in the AOD, optical properties, surface albedo and errors due to the temporal and spatial averaging of the AOD fields. This analysis produced monthly, regional estimates of the clear-sky aerosol radiative effect and its uncertainty, which were combined to produce annual, global mean values of (−6.7 ± 3.9) W m−2 at the top of atmosphere (TOA) and (−12 ± 6) W m−2 at the surface. These results were then used to give estimates of regional, clear-sky aerosol direct radiative forcing, using modelled pre-industrial AOD fields for the year 1750 calculated for the AEROCOM PRE experiment. However, as it was not possible to quantify the uncertainty in the pre-industrial aerosol loading, these figures can only be taken as indicative and their uncertainties as lower bounds on the likely errors. Although the uncertainty on aerosol radiative effect presented here is considerably larger than most previous estimates, the explicit inclusion of the major sources of error in the calculations suggest that they are closer to the true constraint on this figure from similar methodologies, and point to the need for more, improved estimates of both global aerosol loading and aerosol optical properties.
Highlights
Atmospheric aerosol has been held responsible for considerable uncertainty in radiative forcing estimates and the resulting predictions of future climate (Forster et al, 2007)
GlobAEROSOL produced a set of satellite based aerosol products from a range of European satellite sensors, namely: the second Along Track Scanning Radiometer (ATSR-2) on the ERS-2 satellite; Advanced Along-Track Scanning Radiometer (AATSR) and the MEdium Resolution Imaging Spectrometer (MERIS) on ENVISAT; and the Spinning Enhanced Visible-InfraRed Imager (SEVIRI) on the second generation Meteosat geostationary satellites
Of the four instruments included in GlobAEROSOL, only data from AATSR were used in this study, as ATSR-2 data were not available for 2006, SEVIRI does not offer global coverage and there are known quality issues with the GlobAEROSOL MERIS product
Summary
Atmospheric aerosol has been held responsible for considerable uncertainty in radiative forcing estimates and the resulting predictions of future climate (Forster et al, 2007). The diversity of sources and composition of aerosol produces substantial spatial and temporal variability of amount, characteristics and impact on the Earth’s energy budget It is these regional variations that are likely to play a large role in defining regional climate impacts. The spatial pattern of radiative forcing is responsible for some of the largest differences in forcing between different models Reasons for this include the speciation of the aerosols, their prescribed or predicted optical properties and their interactions with clouds. To calculate radiative forcing and quantify the impact of varying sources of uncertainty, we need data including aerosol amount (e.g. optical depth) in both the present day and the pre-industrial era, aerosol vertical profile, aerosol scattering properties (themselves a function of size and composition), and a radiative transfer code
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