Performance assessment of GNSS radio occultation measurements from five missions over China

Global Navigation Satellite System (GNSS) radio occultation (RO) is one of the most crucial observations in atmospheric and climate science. GNSS RO globally produces accurate and long-term stable vertical profiles for essential climate variables such as refractivity and temperature at high vertical resolution in all weather conditions. Currently, various RO satellite constellation programs have been developed by nations and companies, and the growing quantity of RO observations can contribute not only globally but also has the potential to benefit specific regions, such as China. To investigate the potential of RO observation in China, the performance of five operational RO measurements from COSMIC-2, MetOp-B/C, FY-3D/E, Spire and PlanetiQ on data coverage capabilities and quality are assessed by comparing with ERA5 and radiosonde over China. The results of data coverage showed that all RO missions can acquire extensive coverage over China with effective low-altitude penetration capability, whereas MetOp-B/C exhibits some gaps in local time coverage. The results of data quality confirmed that commercial Spire and PlanetiQ are comparable to those of national-led COSMIC-2, MetOp-B/C and FY3D/E, even though Spire exhibited a lower signal-to-noise ratio (SNR). The mean bending angle and refractivity relative differences of all RO measurements are within ±2.9% and ±1.5/0.9% (with respect to ERA5/radiosonde) in the altitude range of 5 to 35 km, respectively, and the corresponding relative standard deviations (SD) are less than 6% and 1.8/2.2%, respectively. Mean temperature and specific humidity differences of all RO measurements are within ±0.8/1.0 K and ± 0.7/1.0 g/kg, respectively, from the near surface to 15 km, with SD of less than 2.1/2.0 K and 1.8/1.7 g/kg. These results can help users further understand the strengths and weaknesses of these RO observations and indicate the significant application potential of numerous high-quality RO profiles from various RO measurements, which is anticipated to enhance numerical weather predictions for China.