Retrieval of top-of-atmosphere fluxes from combined EarthCARE lidar, imager, and broadband radiometer observations: the BMA-FLX product

Abstract. The Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite mission is expected to provide new insights into aerosols, clouds, and radiation. The satellite's payload includes four instruments designed to synergistically retrieve vertical profiles of clouds and aerosols, along with the atmospheric radiation data. This will enable the determination of atmospheric heating rates and top-of-atmosphere radiances and fluxes. This paper focuses on the BMA-FLX processor, specifically created, developed, and validated to retrieve thermal and solar top-of-atmosphere radiative fluxes from longwave and shortwave radiances, measured along track by the EarthCARE Broad-Band Radiometer (BBR) instrument. These radiances are co-registered either at the surface or, in cloudy conditions, at the radiatively most significant vertical layer of the atmosphere (reference level). The Multi-Spectral Imager (MSI) and Atmospheric Lidar (ATLID) on board EarthCARE support cloud identification, while meteorological data from the European Centre for Medium-Range Weather Forecasts provide the surface and atmospheric necessary information. In the BMA-FLX processor, flux is estimated independently for each BBR view using different approaches for the longwave and shortwave radiances. A combined flux, derived from co-registered radiances at the reference level, is provided as the best estimate for a given scene. The radiance-to-flux conversion algorithms have been successfully validated through end-to-end verification using L1 and L2 synthetic data for three EarthCARE orbits. In general, a good agreement is found between the retrieved fluxes and the model truth, with root mean square errors (RMSEs) varying between 7 and 18 W m−2 for the solar fluxes and lower than 6 W m−2 for the thermal fluxes. The BMA-FLX's objective is to achieve radiative closure for EarthCARE with solar and thermal fluxes within 10 W m−2.