| dc.description.abstract | The FORMOSAT-7 satellites, which were launched into space on June 25, 2019, provide higher resolution and quality radio occultation data than FORMOSAT-3. The radio signals are transmitted from the satellites of Global Navigation Satellite System (GNSS) and received by low earth orbit (LEO) satellites. The vertical profiles of bending angle are derived by measuring the refraction of the signals. Radio occultation measurements are helpful for numerical weather prediction (NWP), which can improve the analysis and thus further improve the typhoon track and intensity prediction.
In this study, the global NWP system at CWB (CWB FV3GFS), which utilizes the global model FV3GFS and the GSI 3DEnVar data assimilation system, is used to assimilate observation data. The impacts of bending angle data assimilation on forecasts of typhoons Hagibis, Maysak and Haishen are investigated. Both experiments that assimilate and don’t assimilate bending angle data (denoted by WB and NB, respectively) are performed and they are conducted for 120-h forecast at every 00Z and 12Z. The experiments are initialized at 14 days before the observed cyclogenesis time of each typhoon.
The model results show that the forecasted tracks of all WB and NB experiments are close to the best tracks from JMA, and the track predictions for Maysak and Haishen are significantly improved in their later forecasts. Further comparisons on the forecasted tracks show that the track errors for WB are slightly larger than NB. The track errors for WB are not improved for all typhoons, however the intensity errors are significantly improved in a statistical manner, particularly for the pressure errors. The experiments that their forecasted tracks are closer to the best track are chosen for detailed analysis. The WB analyses after data assimilation for Maysak and Hagibis are closer to the NCEP GDAS/FNL data compared to NB and thus improve the typhoon intensity and structure. For Maysak, stronger wind near the typhoon center at the initial time is produced in WB than NB and is accompanied by moister updrafts, which is favorable for the development of the typhoon. | en_US |