Validating satellite imagery derived cloud optical thickness and albedo with airborne observations from the 2016 - 2018 deployments of the NASA project ObseRvations of Aerosols above CLouds and their intEractionS

The radiative effect of aerosols above clouds (a frequently encountered constellation in the southeast Atlantic Ocean) depends on the albedo of these clouds, which can be calculated from imagery-derived cloud optical properties. The operational retrieval for MODIS on the polar orbiting satellites Aqua and Terra, and the research retrieval for the geostationary SEVIRI satellite are based on a bi-spectral reflectance method, which relies on the radiation calculations made under aerosol-free and plane-parallel assumptions. Meyer et al. implemented a multi-spectral algorithm that retrieves above-cloud aerosol optical thickness along with cloud optical thickness and droplet effective radius, based on two sets assumptions of aerosol optical properties (single scattering albedo, asymmetry parameter, etc.). Which of these is more appropriate, and whether the above-cloud albedo is accurately predicted, can be answered with field missions such as ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS). Three deployments (2016, 2017, 2018) provided direct measurements of the radiative properties for both aerosols and clouds. This study uses the spectral upwelling and dowelling irradiance data from the Solar Spectral Flux Radiometer (SSFR) to evaluate MODIS- and SEVIRI-derived above-cloud albedo, while providing independent retrievals of spectral single scattering albedo and scattering asymmetry parameter from SSFR and the Spectrometer for Sky-Scanning Sun-Tracking Atmospheric Research (4STAR). Our results show that under light aerosol loads and for scattered clouds, the low bias in above-cloud albedo (overall up to 10%) due to spatial inhomogeneity may rival that caused by the aerosol layer.