## 4 Conclusions  
In this study, the 3 DVAR/EnKF hybrid DA was integrated with the multi-resolution global model MPAS for typhoon forecasts with RO data assimilation lasting for two days to improve the initial condition. The hybrid DA consists of 36 ensemble members in GSI to update the background covariance with ensemble forecasts of MPAS at 120 km resolution and the 3 DVAR to minimize the cost function for analysis used for the MPAS forecast at 60-15 km resolution to update analysis in continuous DA cycles at 6 h frequency. This is an approach with dual resolutions in the DA system. At the end of cycled assimilation, free forecasts of a total of 120 h were conducted for the experiments with different sets of GTS and RO data. Local bending angle and refractivity operators in GSI were used for RO data assimilation in the experiments, BND and REF, respectively, which are the same as the experiment GTS that assimilates all operationally available observations (including convectional radiosonde sounding data and satellite radiances) except for the RO data. In this study, we chose the typhoon case Nepertak (2016) to explore the impact of the RO data on typhoon track and intensity prediction, which can be compared with our previous work in [PERSON] et al. (2022) that applies the CWB global model for ensemble forecasts in the hybrid assimilation and the MPAS model for main forecasts. For improving the initial typhoon structure and intensity at the end of RO data assimilation, a dynamic vortex initialization (DVI) scheme that may effectively spin up the initial inner typhoon vortex through cycled model integration was applied to improve the typhoon vortex intensity and structure ([PERSON] et al., 2019). For the DVI, cycled runs of one hour using the forecast model continued to update the  
Figure 18: Geopotential height (contours at an interval of 10 gpm), wind speed (shaded in m s\({}^{-1}\)), and wind vectors (m s\({}^{-1}\)) at 850 hPa at 1200 UTC 26 September 2019 for (**a**) the NCEP FNL data and (**b**) after the first DA cycle. (**c.d**) as in (**a,b**), respectively, but at 0000 UTC 27 September 2019. The initial center position of Tyhoon Mitag from the best track data of CWB and JTWC is indicated by the red typhoon sign and green typhoon sign, respectively. typhoon vortex until the typhoon reached the best track wind and pressure intensity, called V-match and P-match, respectively. The two-way system reasonably captured the track and intensity of Nepartak with a forecast skill similar to the one-way system, both showing forecast improvement at earlier stages (days 1-3), but slightly degraded at later stages, when compared to the prediction using the global reanalysis at the end assimilation. The assimilation of additional RO data improved the track forecasts for REF and BND but has no appreciable impact on the intensity forecasts compared to GTS. Application of the DVI at the end of RO data assimilation largely improved the deepening rate of the typhoon before landfall at Taiwan for both REF and BND, but at cost of slightly increased track errors, especially for REF. Use of the DVI remains to be a preferred option with an acceptable trade-off, especially for GTS and BND. Use of a higher resolution of 60-15-3 km for BND further improved the intensity forecast, especially for the typhoon near landfall at day 4. The DVI at the higher resolution also further improved the intensity forecast with more consistent and stronger deepening rates, but with increased northward track deviations after day 3. The northward track deviations in some experiments were explained in this study using the wavenumber-one PV budget analysis, which is primarily dictated by PV horizontal advection compared to PV vertical advection and differential diabatic heating. The feasibility of the 3 DVAR/EnKF hybrid system (MPAS-GSI) was also illustrated in forecasts of Typhoon Mitag (2019) with more RO data available from FORMOSAT-7/COSMIC-2.