Atmospheric Correction Above Clouds for FY-4A AGRI Based on RTTOV

Developing a rapid and efficient method to mitigate atmospheric interferences above clouds is essential for precise retrieval of cloud physical properties through satellite remote sensing. In this study, radiative transfer for TIROS operational vertical sounder (RTTOV) was employed to generate lookup tables (LUTs) for FengYun-4A (FY-4A) Advanced Geostationary Radiation Imager (AGRI). These LUTs comprised information on atmospheric transmittance, Rayleigh optical depth, and upwelling emission above clouds. A total of 288 images in 2021 were analyzed. Following correction, notable differences in reflectance or radiance were observed between top-of-atmosphere (TOA) and top-of-cloud (TOC) measurements for bands 1, 6, 7, and 11, with average relative biases ranging from 6.94% to 17.60%. Conversely, bands 2, 3, 5, 12, and 13 showed relatively minor variations, with an average relative bias of less than 3.98%. Solar zenith angle (SZA), gas column concentration above clouds, and cloud-top temperature were primary factors influencing the variations. Moreover, rapid correction equations were developed utilizing the least-squares method. The exponential equation yielded the most precise gas transmittance above clouds, while linear and exponential equations were found to be optimal for atmospheric upwelling emission in bands 7 and 11, and bands 12 and 13, respectively. The root-mean-square error (RMSE) between radiance corrected by the equations and the LUTs, consistently stayed below 0.50, with a mean absolute percentage error (MAPE) below 4.03%, and a coefficient of determination (
$R^{2}$
) exceeding 0.95. The rapid correction equations, capable of independently, efficiently, and accurately computing TOC reflectance or radiance, are highly suitable for cloud property inversion.