Assimilation Impact Of Early Formosat-7/Cosmic-2 Gnss Radio Occultation Data With Taiwan'S Cwb Global Forecast System

The FORMOSAT-7/COSMIC-2 Global Navigation Satellite System (GNSS) Radio Occultation (RO) satellite constellation was launched in June 2019 as a successor of the FORMOSAT-3/COSMIC mission. The Central Weather Bureau (CWB) of Taiwan has received FORMOSAT-7/COSMIC-2 GNSS RO data in real time from the Taiwan Analysis Center for COSMIC. With the global numerical prediction system at CWB, a parallel semioperational experiment assimilating the FORMOSAT-7/COSMIC-2 bending angle data with all other operational observation data has been conducted to evaluate the impact of the FORMOSAT-7/COSMIC-2 data. The first seven-month results show that the quality of the early FORMOSAT-7/COSMIC-2 data has been satisfactory for assimilation. Consistent and significant positive impacts on global forecast skills have been observed since the start of the parallel experiment, with the most significant impact found in the tropical region, reflecting the low-inclination orbital design of the satellites. The impact of the FORMOSAT-7/COSMIC-2 RO data is also estimated using the ensemble forecast sensitivity to observation impact (EFSOI) method, showing an average positive impact per observation similar to other existing GNSS RO datasets, while the total impact is impressive by virtue of its large amount. Sensitivity experiments suggest that the quality control processes built in the Gridpoint Statistical Interpolation (GSI) system for RO data work well to achieve a positive impact by the low-level FORMOSAT-7/COSMIC-2 RO data, while more effort on observation error tuning should be focused to obtain an optimal assimilation performance. This study demonstrates the usefulness of the FORMOSAT-7/COSMIC-2 RO data in global numerical weather prediction during the calibration/validation period and leads to the operational use of the data at CWB.Significance Statement The importance of the Global Navigation Satellite System Radio Occultation measurement has been widely acknowledged by the numerical weather prediction community. Following the successful FORMOSAT-3/COSMIC mission, the FORMOSAT-7/COSMIC-2 constellation, consisting of also six radio occultation satellites, was launched on 25 June 2019. As a collaborator of the mission, we prepared early to investigate the anticipated impact of the FORMOSAT-7/COSMIC-2 data on weather prediction as soon as the preliminary data were available. Statistically significant improvement in global forecast skills is clearly shown, with the most significant impact found in the tropics, consistent with the primary goal of the mission. The real-time tropospheric and stratospheric FORMOSAT-7/COSMIC-2 radio occultation data have been officially released since March 2020, available for use in operational weather prediction worldwide.