Quality Assessment of Tianmu-1 GNSS Radio Occultation Refractivity over South China and Surrounding Regions During the 2024 Rainy Season

Tianmu-1 (TM-1) provides global navigation satellite system (GNSS) radio occultation (RO) observations from four navigation systems. To support assimilation over humid South China, TM-1 refractivity was evaluated over South China and surrounding regions for April to September 2024 against ERA5, NCEP FNL, and IGRA2 radiosondes. The results show that the layer-mean differences from the analyses are within ±0.4%, statistically significant but small compared with the spread, and that the total error decreases from about 3% near the surface to 1% at 8 km and remains below 1% in the upper troposphere and lower stratosphere before the spread increases again above 25 km. The data quality remains stable from month to month during the rainy season. The four constellations differ little below 20 km, but GPS and GLONASS show a 25% larger spread at 20–30 km (variance ratio 1.57, F-test p<0.001) and Galileo penetrates less deeply. Superrefractive gradients occur in about 7% of the radiosonde layers near the surface, and superrefraction-aware quality control is recommended below 8 km. These results provide a basis for specifying TM-1 observation errors in regional assimilation.