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Session : COSMIC/FORMOSAT

The Use of FORMOSAT-7/COSMIC-2 GNSS Radio Occultation Data in Numerical Weather Prediction at Central Weather Bureau
Guo-Yuan Lien, Zih-Mao Huang, Chung-Han Lin, Ying-Jhen Chen, Wen-Hsin Teng, Jen-Her Chen, Jing-Shan Hong, Central Weather Bureau

The Global Navigation Satellite System (GNSS) radio occultation (RO) data have been known to be valuable for routine weather forecast. All major operational weather centers around the world assimilate the available GNSS RO data into their numerical weather prediction (NWP) models to improve the three-dimensional initial conditions of the models and thus the model forecasts. Among the RO satellites, FORMOSAT-3/COSMIC was the first satellite constellation providing a large amount of real-time RO data with a good spatial and temporal coverage, which was shown to be very beneficial to the NWP accuracy. Then, FORMOSAT-7/COSMIC-2 (FS7/C2) was launched in 2019 to become a better successor of the previous one. The timely and optimal use of the FS7/C2 GNSS RO data in NWP had become one of the critical tasks at the time at Central Weather Bureau (CWB) of Taiwan.
The early assessment by CWB’s NWP team showed a significant average positive impact of the FS7/C2 data in its operational global NWP model, with the largest impact found in the tropical region, reflecting the low-inclination orbital design of the satellite constellation. The diagnosis using the Ensemble Forecast Sensitivity to Observation Impact (EFSOI) technique estimated the FS7/C2 data impact at about 6 to 8% of the total impact of all the assimilated observation data. These results supported the operational use of the FS7/C2 data at CWB. Afterwards, the impact of the FS7/C2 data assimilation has been studied for specific weather systems such as typhoons and for different NWP systems including a regional model. To further extract useful information of the GNSS RO data in NWP, several different assimilation strategies and configurations have been experimented, and the one specifying observation errors in relative values shows a good impact. In this presentation, we will summarize the current status and achievement of the FS7/C2 RO data assimilation in NWP systems at CWB.

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