Satellite-derived direct radiative effect of aerosols dependent on cloud cover

Aerosols from biomass burning can alter the radiative balance of the Earth by reflecting and absorbing solar radiation1. Whether aerosols exert a net cooling or a net warming effect will depend on the aerosol type and the albedo of the underlying surface2. Here, we use a satellite-based approach to quantify the direct, top-of-atmosphere radiative effect of aerosol layers advected over the partly cloudy boundary layer of the southeastern Atlantic Ocean during July–October of 2006 and 2007. We show that the warming effect of aerosols increases with underlying cloud coverage. This relationship is nearly linear, making it possible to define a critical cloud fraction at which the aerosols switch from exerting a net cooling to a net warming effect. For this region and time period, the critical cloud fraction is about 0.4, and is strongly sensitive to the amount of solar radiation the aerosols absorb and the albedo of the underlying clouds. We estimate that the regional-mean warming effect of aerosols is three times higher when large-scale spatial covariation between cloud cover and aerosols is taken into account. These results demonstrate the importance of cloud prediction for the accurate quantification of aerosol direct effects. Aerosols derived from biomass burning make a significant but poorly quantified contribution to anthropogenic radiative forcing of climate1–3 and may affect regional atmospheric circulation4. The most significant differences between model estimates of the topof-atmosphere direct climate forcing (DCF) are in regions where these aerosols dominate the forcing3. DCF is the change in the topof-atmosphere direct radiative effect (DREtoa) since pre-industrial times and cannot be determined from modern measurements alone5. Both DREtoa and the absorption within the atmosphere (DREatm) are sensitive to aerosol optical properties (chiefly aerosol optical thickness τ , absorption and size distribution)6, and also to the albedo of the underlying surface7,8. In the absence of clouds, DREtoa is negative over the ocean owing to its low surface albedo even when the aerosol is strongly absorbing9. However, when absorbing aerosol layers are located above clouds, DREtoa can be positive2,10. Although a few modelling studies have attempted to quantify the regional effects of clouds on the DRE (for example, ref. 10), here we use spaceborne lidar observations of aerosols above clouds, together with observed cloud optical properties, to quantify the aerosol DRE and the effect of clouds on it. Previous intensive observational studies of aerosols derived from biomass burning, conducted during field campaigns over North and South America11–13 and Africa14,15, are limited either in time or space. Passive remote sensing of aerosol optical properties is routinely conducted at numerous surface sites across the globe (for example, the AERONET project16) and from satellites17–19,