FY-4B/GIIRS AVP Weak-Constraint-Enhanced GNSS Water Vapor Tomography over Hong Kong

The vertical distribution of atmospheric water vapor plays a key role in the development of heavy precipitation and convective systems, yet conventional GNSS water vapor tomography remains constrained by uneven ray coverage and insufficient voxel support. To address this limitation, this study developed a three-dimensional GNSS water vapor tomography framework strengthened by weak constraints derived from the FY-4B/GIIRS atmospheric vertical profile product (AVP). Because the AVP provides vertically resolved humidity information, it has the potential to supplement the weakly constrained structure of regional GNSS tomography, particularly in the middle and upper troposphere. To evaluate the effectiveness of this strategy, four comparative experiments were designed under two background fields, with and without AVP weak constraints. The results showed that AVP-induced analysis increments and error reduction were concentrated mainly above 3 km and were more evident under the GFS background. Among the four schemes, Tomo-IV achieved the best overall performance, with relative RMSE of 26.0% for the full altitude range. Positive spatial gains were also observed in the regional ERA5-referenced evaluation under the GFS background. Unlike previous studies that incorporated FY-series satellite data primarily as geometric supplements to the GNSS observation system, the present framework introduces the FY-4B/GIIRS AVP as voxel-level weak constraints without modifying the native GNSS observation geometry, aiming to improve reconstruction stability and vertical moisture structure in the middle and upper troposphere. These results confirm the effectiveness of this strategy.