An Improved Scheme for FY-3E/HIRAS-II Radiance Simulation at Large Scan Angles

To address the bias in radiative simulation caused by the horizontal inhomogeneity of atmospheric parameters under large scan angles for the HIRAS-II (Hyperspectral Infrared Atmospheric Sounder-II) onboard the Fengyun-3E satellite, this study proposes a method for constructing slant-path atmospheric parameter profiles along the satellite’s line-of-sight (Exp.2). In contrast to the conventional method (Exp.1) based on the assumption of horizontally homogeneous vertical atmospheric profiles, this method accurately calculates the intersection points between the satellite’s line-of-sight and the various altitude layers of the ECMWF reanalysis data version-5 (ERA5). It employs a hybrid interpolation algorithm combining inverse distance weighting and spline interpolation to obtain a continuous distribution of atmospheric parameters along the slant path, thereby accounting for the actual observation geometry of the satellite. The results show that when the satellite zenith angle exceeds 30°, the simulation differences between Exp.2 and Exp.1 increase significantly. Specifically, in the CO2 absorption band, the differences between the two methods are mainly concentrated between −0.1 K and 0.1 K, whereas in the water vapor absorption band, this range expands to between −0.5 K and 0.5 K. Notably, biases are concentrated near the scan edges and exhibit a strong latitudinal dependence, with high-latitude areas showing more prominent deviations due to steeper atmospheric parameter gradients and asymmetric orbital geometry. Moreover, a comparison with the measured satellite-observed brightness temperature further demonstrates that Exp.2 effectively reduces simulation biases near the scan edge. The mean bias is reduced by up to 0.1 K in water vapor absorption channels, a more significant improvement compared to the 0.03 K reduction in CO2 absorption channels. These results indicate that the proposed slant-path profile construction method significantly enhances the accuracy and reliability of infrared hyperspectral radiative transfer forward simulations under complex observation geometries by providing a more realistic representation of the three-dimensional slant-path radiative transfer process. This advancement holds important implications for improving the atmospheric correction of remote sensing data.