We investigated whether Top-of-Atmosphere Shortwave (TOA SW) anisotropy—essential to convert satellite-based instantaneous TOA SW radiance measurements into TOA SW fluxes—is sensitive to cloud-top effective radii and cloud-topped water vapor. Using several years of CERES SSF Edition 4 data—filtered for overcast, horizontally homogeneous, low-level and single-layer clouds of cloud optical thickness 10—as well as broadband radiative transfer simulations, we built refined empirical Angular Distribution Models (ADMs). The ADMs showed that anisotropy fluctuated particularly around the cloud bow and cloud glory (up to 2.9–8.0%) for various effective radii and at highest and lowest viewing zenith angles under varying amounts of cloud-topped moisture (up to 1.3–6.4%). As a result, flux estimates from refined ADMs differed from CERES estimates by up to 20 W ${\mathrm{m}}^{-2}$ at particular combinations of viewing and illumination geometry. Applied to CERES cross-track observation of January and July 2007—utilized to generate global radiation budget climatologies for benchmark comparisons with global climate models—we found that such differences between refined and CERES ADMs introduced large-scale biases of 1–2 W ${\mathrm{m}}^{-2}$ and on regional levels of up to 10 W ${\mathrm{m}}^{-2}$. Such biases could be attributed in part to low cloud-top effective radii (about 8 μ$\mathrm{m}$) and low cloud-topped water vapor (1.7 kg ${\mathrm{m}}^{-2}$) and in part to an inopportune correlation of viewing and illumination conditions with temporally varying effective radii and cloud-topped moisture, which failed to compensate towards vanishing flux bias. This work may help avoid sampling biases due to discrepancies between individual samples and the median cloud-top effective radii and cloud-top moisture conditions represented in current ADMs.

The Broad-Band Radiometer (BBR) instrument on the EarthCARE satellite will provide accurate outgoing solar and thermal radiances at the Top of the Atmosphere (TOA) obtained in an along track configuration at three fixed viewing directions (nadir, fore and aft).The operational BMA-FLX product on top-of-atmosphere radiative fluxes, is based on a radiance-to-flux conversion algorithm mainly fed by the unfiltered broad-band radiances, obtained in the BM-RAD product, auxiliary data from EarthCARE L2 cloud products and modelled geophysical databases. The conversion algorithm models the angular distribution of the reflected solar radiation and thermal radiation emitted by the Earth-Atmosphere system, and returns geometry independent flux estimates to be used for the radiative closure assessment of the Mission.Different methodologies are employed for the solar and thermal BBR ADMs. Models for SW radiances are created for different scene types and constructed from Clouds and the Earth’s Radiant Energy System (CERES) data using a feed-forward back-propagation artificial neural network (ANN) technique. The LW angular models are derived through multiple regressions on brightness temperatures and brightness temperature differences of the multispectral imager (MSI) 10.8 µm and 12 µm channels, and corresponding LW fluxes obtained by using a large database of LibRadtran and SBDART radiative transfer simulations.Both retrieval algorithms exploit the multi-viewing capability of the BBR by applying the radiance to flux conversion algorithms to each of the BBR views, which have been previously collocated at a reference level in order to minimize parallax effects. The reference height where the three BBR measurements are co-registered corresponds to the height where most reflection or emission takes place and depends on the spectral regime. LW observations are co-registered at the cloud top height while SW reference height is instead selected by minimizing the flux differences between nadir, fore and aft fluxes. The derived fluxes from the collocated views are then combined into a single flux value at the selected reference level.Verification of the algorithms has been carried out using the 3 test scenes developed by the EarthCARE team using the Environment Canada and Climate Change’s Global Environmental Multiscale model (GEM). The BBR solar and thermal flux retrieval algorithms have been successfully employed to retrieve radiative fluxes over the 3 test scenes. Comparisons with the true fluxes from the GEM model provide RMSE < 5 W/m² for the LW fluxes and < 15 W/m² for the SW fluxes.