Evaluating the Impact of COSMIC-2 RO on Regional Numerical Weather Prediction using a High-Resolution Hybrid 3DEnVar System at CWB

Constellation for Observing System for Meteorology, Ionosphere and Climate (COSMIC) was a joint Taiwan-U.S. satellite mission that makes use of the radio occultation (RO) measurement technique. Since its launch in 2006, COSMIC RO data has been shown to be extremely valuable for numerical weather prediction, such as improving the prediction of tropical cyclogenesis and reducing the typhoon track error. The follow-on COSMIC-2 mission was launched on 25 June 2019, and is currently going through preliminary testing and evaluation. After it is fully deployed, COSMIC-2 is expected to provide 6,000 GNSS (Global Navigation Satellite System) radio occultation (RO) profiles per day from 40S to 40 N.

In this study, we conduct a preliminary evaluation of COSMIC-2 GNSS RO data on heavy precipitation events associated with typhoon and southwesterly monsoon flows based on the operational NWP system of the Central Weather Bureau (CWB) in Taiwan. The COSMIC-2 GNSS RO data are assimilated using a dual-resolution hybrid 3DEnVare system with a 15-3 km nested-grid configuration. In the 15km resolution domain, flow-dependent background error covariances (BECs) derived from the perturbation of ensemble adjustment Kalman filter (EAKF), will be used to conduct hybrid 3DEnVar analysis. In the 3km resolution domain, the 15km resolution flow-dependent BECs will be inserted to the 3km grid using a Dual-Resolution (DR) technique, and then combined with 3km resolution static BECs, to perform the high-resolution 3DEnVar analysis. The performance of the CWB operational NWP system on quantitative precipitation forecast of significant precipitation events with and without the assimilation of COSMIC-2 GNSS RO data will be evaluated.