Radiative Flux Es- timation from a Broadband Radiometer Using Synthetic Angular Models in the Earth- CARE Mission Framework. Part I: Methodology.

The instantaneous top-of-atmosphere (TOA) radiance-to-flux conversion for the Broad-Band Radiometer (BBR) on-board the Earth Clouds Aerosols and Radiation Explorer (Earth- CARE) is assessed by developing theoretical angular distribution models (ADMs) specifically designed for the instrument viewing configuration. This paper validates the BBR ADMs by comparing derived flux estimates against flux retrievals obtained from the Clouds and the Earth’s Radiant Energy System (CERES) Terra models. A CERES BBR-like data base is employed in the assessment which is the optimum data set to validate the BBR algorithms and to determine the benefits of the multi-angular conversion procedures in the BBR instrument. A direct comparison between models proposed for BBR and CERES is difficult to perform due to the differences in the design of the instruments. Nevertheless, the validation of theoretical results with empirical data is essential in order to prepare the conversion algorithms prior to the launch of EarthCARE. This paper demonstrates that the application of linear combination method results less advisable when outgoing radiances do not follow the response modelled in the radiative transfer calculations. The effective radiance averaged model outperforms all other models in terms of the coefficient of variation of the root mean square errors (CVRMS) in the validation study of the SW regime (clear-sky 1.9%; cloudy 7.1%), while the effective radiance along-track model obtains the best comparisons for the LW regime (clear-sky 1.4%; cloudy 1.5%). The evaluation of the multi-angular models with
scenes with high anisotropy shows that the multi-view flux conversion algorithm improves
the CERES ADM results when CERES flux discrepancies are higher than 4 W m−2 in the LW domain and SW clear-sky scenes, and higher than 20 W m−2 in SW targets with cloudy conditions.