## 5 Albedo Inversion and Analysis  
### Application of FY-2G Albedo Retrieval  
The FY-2G observation data of 4 days in 2019 were selected for albedo retrieval in Hefei, which were the 24 th day, the 107 th day, the 228 th day, and the 302 nd day. On these days, there was less cloud cover over Hefei throughout the day. Figure 11 shows the average angle distribution of each image in these 4 days. The coordinates in the figure are polar coordinate systems, the circumference represents the relative azimuth angle, and the radius is the zenith angle. The \"inverted triangle\" symbol represents the observation zenith angle, and the \"dot\" symbol represents the solar zenith angle. It can be seen from the diagram that the observation zenith angle range of FY-2G in Hefei is relatively fixed, generally approximately 40\({}^{\circ}\), and the solar zenith angle and relative azimuth angle change over a large range, which meets the requirement of the BRDF model parameter fitting. Figure 10: The proportion of AFX numbers in each archetype. Figure 9: BRDF shape distribution with observation zenith angle based on BRDF archetypes, (**a**) Principal plane (PP), (**b**) Cross-principal plane (CPP). The top of atmospheric (TOA) reflectivity received by the satellite needs to be atmospherically corrected to obtain the true surface reflectance. FY-2G does not have aerosol products, and the MODIS MOD04_L2 aerosol product for the corresponding times was chosen as the aerosol input parameter for the atmospheric correction. The average aerosol optical thicknesses in Hefei corresponding to these four different times at 550 nm are 0.4100, 0.3372, 0.2967, and 0.9498. With the known aerosol optical parameters, the real surface reflectance can be obtained by using the Second Simulation of the Satellite Signal in the Solar Spectrum (65) radiation transmission model [34] to remove atmospheric interference. Since FY-2G observes the Earth once an hour, multiple sets of multiangle reflectance data can be obtained for multiple observations in the same area within a day so that the real surface albedo of the day can be retrieved according to the BRDF model. The \(S\) is calculated according to the local latitude and the observation time. The \(S\) value will not change much in one day in Hefei, and the noon mean values of these four days are 0.1638, 0.1318, 0.1308, and 0.1506. Figure 12 shows the albedo retrieved using FY-2G satellite data based on the BRDF archetypes. The four images in Figure 12 from left to right are the 24 th, 107 th, 228 th, and 302 nd days of 2019, and the time is at noon of the day. The image sequence in the subsequent images is arranged in this time sequence and is not described one by one. The ranges of albedo in the figure are 0.0175\(\sim\)0.1431, 0.0269\(\sim\)0.1295, 0.0350\(\sim\)0.1409, and 0.0031\(\sim\)0.1820. The average surface albedos per day are 0.0779, 0.0822, 0.0692, and 0.0832. The adjustment coefficients A and B are shown in Figure 13. The average values of A are 1.3346, 1.2199, 0.9898, and 1.2216, and the average values of B are \(-\)0.0662, \(-\)0.0387, \(-\)0.0327, and \(-\)0.0415 corresponding to the four different periods in Hefei. The loss value in the Huber loss regression fitting is used to evaluate the fitting error. The smaller the loss value is, the better the fitting result. The fitting losses to each pixel under the optimal  
Figure 11: Polar coordinate diagram of angle distribution on 24 th, 107 th, 228 th, and 302 nd days in 2019. BRDF archetype are shown in Figure 14. When the loss value is large, the fitting result is inaccurate, so the abnormal points with a loss greater than 0.01 are removed during inversion. The average fitting losses in Hefei are 0.0005, 0.0015, 0.0010, and 0.0023. Using the obtained eight BRDF archetypes, the daily fast inversion of the land surface albedo in Hefei, Anhui was realized, and the inversion fitting error met the requirements. Figure 15 shows the proportion of each class in the eight BRDF archetypes during the albedo inversion of the Hefei area.