Aerosol above cloud, optical depth, and direct radiative effect in the southeast Atlantic

When aerosol overlie bright clouds, their radiative impact on the incident light can be either positive or negative. To better constrain this quantity, we have measured aerosols above clouds during field campaigns spanning 3 years in the southeast Atlantic as part of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES). We present here a summary of the above cloud aerosol optical depth, and the related direct radiative effect on climate. We use aerosol optical depth measured directly from sunlight attenuation using the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) and compare to measurements from the active remote sensing High Spectral Resolution Lidar-2 (HSRL-2). We present a combination of measurements from all 3 deployments during peak biomass burning season. In addition, we use a novel parameterization of direct aerosol radiative effect, based on airborne irradiance measurements from the Solar Spectral Flux Radiometers (SSFR) in scenes with aerosol above clouds. We have observed average above cloud aerosol optical depth at 500 nm ranging from 0.28 to 0.37, with the maximum in August 2017. Initial estimates show an average instantaneous radiative effect of 25.8 W/m² from these aerosols, with a maximum between 16°S and 14°S for September 2016.