Comprehensive Evaluation on Atmospheric Motion Vectors from Fengyun-4B Geostationary Satellite and Their Application in the South China Sea Monsoon Onset

Although the Chinese new-generation Fengyun-4B (FY-4B) geostationary satellite Atmospheric Motion Vector (AMV) products became operational in June 2022, their accuracy and utility remain largely unexamined. This study comprehensively evaluates FY-4B AMV products for August and October 2023, as well as January and April 2024, exploring their application in monitoring the South China Sea Summer Monsoon (SCSSM) onset. The results indicate that AMV products derived from the upper-level water vapor absorption channel (AMV_WV) and the infrared channel (AMV_IR) demonstrate high accuracy when compared with ERA5 reanalysis data. The root mean square error (RMSE) is mostly between 4.5 m s^–1 and 6.4 m s^–1, with coefficients of determination (R²) values ranging from 0.7 to 0.8, indicating the overall reliability of FY-4B AMVs. The observation errors of AMVs exhibit significant vertical structure characteristics. Specifically, the AMV_WV products demonstrate superior accuracy above 350 hPa, while the AMV_IR products exhibit reduced errors in the layers between 200–500 hPa and 700–950 hPa. Spatially, most areas exhibit low observation errors for AMVs, while clear-sky weather and deep convective cloud systems can increase errors. A lack of clouds or water vapor may reduce the number of observation samples in some areas, leading to unstable RMSE performance, which is particularly evident for AMV_WV RMSE around 25°–30°N in January and near 25°S in August. Deep convective cloud systems can influence AMV retrieval results, leading to systematic observation errors, especially for the infrared channel. Additionally, AMV_WV is more reliable during the daytime, with a lower RMSE compared to nighttime, while AMV_IR exhibits a diverging diurnal variation pattern. Finally, the FY-4B AMV_WV products were applied to monitor the SCSSM event in 2024. Significant zonal wind direction reversal characteristics were observed in key regions around the onset date, indicating that AMVs can serve as effective indicators for monitoring the SCSSM onset.