Global all-sky direct radiative forcing of anthropogenic aerosols

The solar direct radiative effect (DRE) is the mean radiative flux perturbation due to the presence of aerosols (both natural and anthropogenic), while the aerosol direct radiative forcing (DRF) is the anthropogenic component of DRE. To date, all (but one) measurement-based estimations of all-sky DRF are computed by scaling the clear-sky DRF by the complement of the cloud fraction. Consequently, these estimations implicitly assume that aerosols in cloudy regions do not contribute to DRF, but this assumption is not valid if there are absorbing aerosols above the clouds or if the cloud optical depth is very thin. We use CERES Synoptic Radiative Fluxes and Clouds (SYN) product and Goddard Chemistry Aerosol Radiation and Transport (GOCART) model to provide the first observation-based all-sky DRF that accounts for anthropogenic aerosols located above clouds. Specifically, we partition the all-sky DRE from CERES SYN into natural and anthropogenic portion based upon the aerosol compositions (natural and anthropogenic) provided by GOCART model. We derive the partition factor by running radiative transfer model first with all aerosol compositions (natural and anthropogenic) from GOCART, then with only natural aerosol compositions from GOCART. Both runs are for all-sky conditions with cloud properties and surface albedos from CERES SYN product. Our results indicate that the global all-sky DRF differs from the clear-sky scaled DRF by 10 to 20% and the regional all-sky DRF differs from the clear-sky scaled DRF by up to 100%.