Cloud Radiative Effect Changed Due to Shading from Above Cloud Absorbing Aerosol in the South East Atlantic

We present measurements from the NASA ObseRvations of CLouds above Aerosols and their intEractionS (ORACLES) field campaign, focused on the South East Atlantic seasonal biomass burning aerosol layers which overlie semi-permanent marine stratocumulus clouds. Airborne measurements by the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) and the Solar Spectral Flux Radiometers (SSFR) are used to retrieve cloud optical properties, and to calculate its cloud radiative effect (CRE) shaded by the overlying aerosol layer. We used cloud-transmitted solar light measured with the hyperspectral instruments 4STAR sampling radiance with 2° field of view directly at zenith below clouds and SSFR sampling irradiance with a hemispherical field of view to determine flight level albedo. We present calculated CRE based on (i) retrieved cloud optical thickness, effective radius, and thermodynamic phase from zenith-viewing scattered light measurements under stratocumulus clouds and (ii) measured properties of the overlying aerosol layer, with varying aerosol optical depth. We use a retrieval technique that takes advantage of spectral features in transmitted light instead of traditional reflectance-based retrievals. This transmittance-based retrieval of cloud optical properties has the added advantage of being less sensitive to typical biases induced by aerosol layers above clouds in reflectance-based retrievals. To account for any biases, we use accurate measurements and retrievals of the overlying aerosol optical properties (aerosol optical depth, single scattering albedo, and asymmetry parameter) in addition to cloud-transmitted light to calculate the CRE below cloud. We have observed that the overlying aerosol layer acts to reduce the top of atmosphere and surface CRE by reducing the incident radiative energy at cloud top, i.e. shading the cloud. We present an early estimate of the CRE based on measured values and calculate the impact of the aerosol’s shading on calculated CRE.