Quality Assessment and Assimilation of Tianmu-1 GNSS Radio Occultation Refractivity Observations: A Preliminary Study

Global Navigation Satellite System (GNSS) radio occultation (RO), owing to its capability to provide high vertical resolution, high accuracy, calibration-free, and all-weather atmospheric observations, has been widely used in numerical weather prediction (NWP) and climate studies. As China’s first commercial GNSS RO constellation supporting all major GNSS systems, Tianmu-1 (TM-1) offers promising observations. However, its data quality and assimilation performance in NWP remain underexplored. This study first evaluates the TM-1 neutral atmospheric refractivity and bending angle profiles collected in January 2024. Compared with the European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ECMWF-ERA5), refractivity fractional differences at 5–30 km have mean and standard deviation within ±0.15% and 1.31%, while bending angle differences are within ±0.55% and 1.99%. Radiosonde comparisons over 0–20 km show refractivity differences within ±0.19% and 1.93%, and bending angle differences within ±0.12% and 5.06%. Larger errors are mainly confined to the lower troposphere and low latitudes, with only minor variations across GNSS constellations. After validating data quality, TM-1 refractivity observations are assimilated using the Weather Research and Forecasting (WRF) model and WRFDA 3DVAR system to assess their impact on regional analyses and short-range forecasts over China. Model outputs are validated against ERA5 reanalysis and radiosonde observations. The results show that assimilating TM-1 refractivity data leads to root mean squared error (RMSE) reductions of ~5–10% for temperature analyses and forecasts in the mid-to-upper troposphere and near the surface, and ~5% in specific humidity in the lower troposphere. Wind impacts are mixed, with RMSE improvements ~2–5% above 600 hPa and degradation in the lower troposphere. Overall, this preliminary study confirms the high quality of TM-1 GNSS RO refractivity data and demonstrates its promising contribution in complementing current operational RO assimilation for regional NWP.