Retrieval of Background Surface Reflectance using Geo-Kompsat-2B/GEMS data

Surface reflectance can be defined as the ratio of solar energy incident to surface reflection. This is calculated by removing the scattering and absorption effects of the atmosphere using the radiative transfer model (RTM), and many missing values occur due to the influence of clouds. However, since surface reflectance is an essential input data onto aerosol output like Aerosol Optical Depth (AOD), surface reflectance without missing values is required. So, in this study, we performed retrieval of background surface reflectance (BSR) using Geo-Kompsat-2B (GK-2B) / Geostationary Environment Monitoring Spectrometer (GEMS) data. BSR is an estimate of the surface reflectance of a particular observation condition (solar zenith angle, viewing zenith angle, etc.), and can avoid the dilemma of calculation antecedent relationships between AOD-Surface reflectance. BSR is mainly calculated using minimum reflectance method, but the BSR calculated through minimum reflectance method is not a reflectance considering the observation conditions and the anisotropic reflectance of the surface. Therefore, in this study, the Bi-directional Reflectance Distribution Function (BRDF) model was used to calculate the realistic BSR considering the anisotropic reflection characteristics of the surface. Surface reflectance, which is an input variable for BRDF modeling, was calculated based on the Look-Up Table (LUT) generated using Second Simulation od Satellite Signal in the Solar Spectrum (6SV) RTM, and interpolation was performed for the LUT using the 6-d interpolation technique. For the BRDF model, the Roujean model (Roujean et al., 1972), which is a semi-empirical model, was used, and the optimal synthesis cycle of the BRDF model was adopted in consideration of the characteristics of the GEMS satellite observed once an hour. To evaluate the accuracy of BSR, the simulated BSR through BRDF model and the observed surface reflectance were compared, and it was confirmed that the BSR showed higher accuracy than the minimum reflectance method. It is judged that using the BSR calculated through this study will greatly affect the accuracy of the aerosol output of GEMS satellites including AOD. The GEMS data used in this study were provided as GEMS Algorithm Team for only validation and improvement research purposes.