Estimating the Anisotropic Factor of Angular Distribution Models from Radiative Fluxes

Global climate change has aroused widespread concerns in society regarding the sustainable development of human beings. The Earth’s radiation budget (ERB) at the Top-of-Atmosphere (TOA) includes incident solar radiation, Earth-reflected shortwave radiation, and outgoing longwave radiation. The accuracy of the existing spaceborne instruments still cannot meet the measurement requirement of the Earth’s TOA shortwave and longwave radiation. In recent years, several novel observing platforms and sensors have been proposed for ERB. Hitherto, most of those concepts for ERB are still in the phase of design and development, and studies have been mainly carried out based on simulations. Simulating the sensor-measured signals of the proposed novel platforms, sensors or constellations could help to optimize the sensor parameters, determine the number of satellites in the constellation, explore their potentials in characterizing the ERB. The anisotropic factor, depicting the anisotropy of Earth’s radiation, is essential in the simulation. However, developing angular distribution models involves complex procedures of data preparation, processing, and modeling. This study, targeting at simplifying the procedure of simulating the signals of ERB sensors, proposed an approach of estimating the longwave anisotropic factors directly from the Earth’s radiative fluxes. The approach was implemented with CERES data and the neural network algorithm. Models were developed for 10 scene types based on Earth’s surface types. Results showed that the longwave anisotropic factors were accurately estimated with the correlation coefficient (r) varying between 0.85 and 0.98 and MAPE within 1.20% for the test dataset. With the estimated anisotropic factors, the sensor-measured radiances were accurately retrieved with r=1.00 and MAPE=0.53%. Therefore, the proposed approach is promising in accurate and efficient simulations of novel ERB platforms and sensors like the Moon-based Earth Radiation.