Revisiting satellite radiative flux computations at the top of the atmosphere

Most satellite observations of radiative fluxes at the top of the atmosphere (TOA) are at narrow spectral intervals and at particular viewing angles. Critical elements in the formulation of TOA shortwave (SW) radiative fluxes are (1) the transformation from narrowband to broadband values (n/b) and (2) the application of angular distribution models (ADMs) to correct for anisotropy. In this article, the n/b transformations are based on theoretical simulations with a radiative transfer model Moderate Resolution Atmospheric Transmission (MODTRAN) 3.7 using land classification types based on the International Geosphere-Biosphere Programme (IGBP) scheme and a range of realistic atmospheric conditions. The newly developed ADMs are a combination of MODTRAN-3.7 simulations and the Clouds and the Earth's Radiant Energy System (CERES)-observed ADMs. To evaluate the impact of the proposed corrections, they are implemented with observations from the Spinning Enhanced Visible Infrared Imager (SEVIRI) on the Meteorological Satellite (METEOSAT) 8 to derive TOA fluxes and compared to similar quantities from CERES. It is shown that the estimated TOA radiative fluxes have -3% bias and 7% root mean square error (RMSE) when compared with CERES observations at a monthly timescale.