Quantifying GNSS Tropospheric Wet Delay Responses During Tropical Cyclone Passage Over Southeastern Africa

Tropical cyclones are among the most destructive atmospheric systems, yet their associated moisture evolution remains poorly constrained in data-sparse regions such as Southeastern Africa. This study investigates Global Navigation Satellite System (GNSS)-derived zenith wet delay (ZWD) variability during five tropical cyclones between 2015 and 2023 using continuously operating GNSS observations and best-track cyclone data. A cyclone-centred composite framework is used to characterise ZWD evolution relative to closest approach. Results show pronounced cyclone-induced ZWD enhancements, with peak responses approaching 370 mm and systematically stronger variability after cyclone passage than before. However, permutation-based significance testing indicates that the observed temporal asymmetry is not distinguishable from stochastic variability, implying that robust precursor signatures are not consistently present in the analysed events. Spatial analysis further reveals only weak dependence of ZWD response magnitude on cyclone–station separation, suggesting that storm structure and mesoscale moisture transport dominate over simple radial distance controls. GNSS-derived ZWD shows physically consistent signatures of cyclone-related atmospheric moisture redistribution. However, it does not exhibit consistent precursor behaviour for cyclone evolution in Southeastern Africa in this study.