Quality Assessment and Assimilation of Tianmu-1 GNSS Radio Occultation Refractivity Observations: A Preliminary Study
Jiafeng Li, Cuixian Lu, Yuxin Zheng, Wei Ban, Xiaohong Zhang
Abstract
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.