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. The direct radiative effect (DRE) of aerosols aboveclouds has been found to be significant over the south-east AtlanticOcean during the African biomass burning season due to elevated smokelayers absorbing radiation above the cloud deck. So far, globalclimate models have been unsuccessful in reproducing the high DREvalues measured by various satellite instruments. Meanwhile, theradiative effects by aerosols have been identified as the largestsource of uncertainty in global climate models. In this paper, threeindependent satellite datasets of DRE during the biomass burningseason in 2006 are compared to constrain the south-east Atlanticradiation budget. The DRE of aerosols above clouds is derived from thespectrometer SCanning Imaging Absorption spectroMeter forAtmospheric CHartographY (SCIAMACHY), the polarimeter Polarization andDirectionality of the Earth's Reflectances (POLDER), andcollocated measurements by the spectrometer Ozone MonitoringInstrument (OMI) and the imager Moderate Resolution ImagingSpectroradiometer (MODIS). All three datasets confirm the high DREvalues during the biomass season, underlining the relevance of localaerosol effects. Differences between the instruments can be attributedmainly to sampling issues. When these are accounted for, theremaining differences can be explained by a higher cloud opticalthickness (COT) derived from POLDER compared to the other instrumentsand a neglect of aerosol optical thickness (AOT) at shortwave infrared(SWIR) wavelengths in the method used for SCIAMACHY and OMI–MODIS. Thehigher COT from POLDER by itself can explain the difference found inDRE between POLDER and the other instruments. The AOT underestimationis mainly evident at high values of the aerosol DRE and accounts forabout a third of the difference between POLDER and OMI–MODIS DRE. Thedatasets from POLDER and OMI–MODIS effectively provide lower and upperbounds for the aerosol DRE over clouds over the south-east Atlantic,which can be used to challenge global circulation models (GCMs). Comparisonsof model and satellite datasets should also account for samplingissues. The complementary DRE retrievals from OMI–MODIS and POLDER maybenefit from upcoming satellite missions that combine spectrometerand polarimeter measurements.

Comparison of south-east Atlantic aerosol direct radiative effect over clouds from SCIAMACHY, POLDER and OMI–MODIS