An optimal sequential physical retrieval system for retrieving high-accuracy diurnal atmospheric gases from FY-4B/GIIRS: Theory, algorithm and evaluation

The Geostationary Interferometric Infrared Sounder (GIIRS) onboard the FY-4B satellite is the world's first and currently the only operational hyperspectral thermal infrared sounder in geostationary orbit, with the unique advantage of continuously scanning the atmosphere over East Asia on an hourly basis during both daytime and nighttime. Compared to previously established low-Earth orbit satellite sounders, developing and applying Level 2 atmospheric products from FY-4B/GIIRS are still in the exploratory stage. In this study, we present an optimal sequential physical retrieval system (OSPRS) for retrieving high-accuracy atmospheric strong absorbers, including water (H₂O), ozone (O₃) and carbon monoxide (CO) from FY-4B/GIIRS. OSPRS first selects a subset of sensitive spectral channels for each variable based on column- and pressure-related sensitivity. It then determines the optimal retrieval sequence consisting of multiple retrieval steps, aiming to reduce the nonlinearity of each inversion problem and the influence of interfering variables on the primary retrieval targets. Finally, OSPRS employs the optimal estimation method as the retrieval operator to perform the retrieval at each step, outputting the profiles of the primary retrieval targets and critical scientific diagnostic information. We confirm the improved accuracy of OSPRS through Observing System Simulation Experiments (OSSE). We compare OSPRS with existing products and evaluate them based on high-quality in situ data from the Integrated Global Radiosonde Archive H₂O, the ground-based Pandonia Global Network O₃, and solar absorption Fourier transform infrared CO measurements. The results show that the mean absolute error, linear fitting slope, and correlation coefficient between OSPRS's H₂O, O₃ and CO and in-situ or ground-based measurements are superior to those of existing products. This study is dedicated to providing the community with high-quality atmospheric products retrieved from FY-4B/GIIRS and promoting the research and application of GIIRS in numerical weather forecasting, atmospheric environment, and other related fields.