## 3 Simulations and Discussion
### Impact on Forecast of Typhoon Mittag (2019)  
The RO impact on typhoon forecast was also investigated by an extension to other cases occurring at later times when more RO data became available from FORMOSAT-7/COSMIC-2, a follow-on mission of FS3/C. For an illustration of the MPAS-GSI system performance, we chose one case, Typhoon Mittag (2019), that moved toward Taiwan at earlier stages, similar to Nepertalk, but then took a north turn when nearing Taiwan. The experiment with the same model configurations as for Typhoon Nepertalk was conducted with DA for 12 h till 0000 UTC 27 September 2019 and then followed by a forecast for 5 days. There are 3924 RO soundings available during the DA period. Figure 17 shows the forecasted tracks, track errors, and central sea-level pressure for NoDA, GTS, BND, and BND_RV for this case. All the forecasted tracks for the four experiments deviated northward from the best track since the initial time, but the GTS track becomes more eastward away from the observed (Figure 17a) leading to larger errors at the end forecast than both BND and BND_RV (Figure 17b). Note that NoDA using the NCEP FNL after the DA gives the largest track deviations, despite the fact that its initial typhoon center is closer to the observed. The vortex relocation in BND_RV enables the initial typhoon to closely collocate with the best track position, but the later track evolution is similar to that without the relocation in BND and both give similar track errors. For BND_RV with the V-match, the initial central sea-level pressure is improved but does not reach the best-track maximum intensity, which leads to a somewhat enhanced typhoon intensity only in the first two days. The later evolution of typhoon intensity for the four experiments, in general, is similar with pronounced underprediction (Figure 17c). It is probably due to the further northward track of the simulated typhoon that passes over the colder ocean as compared to the track of the observed typhoon over the warmer sea surface (not shown). Further analyses are needed to identify the factors for the slower typhoon intensification compared to the best track data of the Joint Typhoon Warning Center (JTWC). Although this example is not very stimulating for all forecasts, it essentially shows the sole impact of RO data assimilation with and without the DVI. From the forecast results in Figure 17, a combination of both RO data assimilation and DVI still produces large eastward track deviations, despite the fact that the later track forecast has been improved. It is interesting to understand the performance of the BND experiment for this case. The geopotential height and horizontal wind at 850 hPa at 1200 UTC 26 September 2019 from the NCEP FNL dataset and the BND experiment after the first DA are shown in Figure 18. Compared to the FNL analysis, the location of the initial typhoon center in the BND experiment after the first DA is very close to the best track positions from CWB and JTWC (Figure 18b). After 12 h, the typhoon center shown by the FNL data deviated somewhat eastward from the best track positions of CWB and JTWC (Figure 18c); however, the typhoon center has a northeast shifting after the third DA cycle (Figure 18d) in association with a somewhat stronger and larger circulation compared to the global FNL analysis. It appears that the wind and associated zonal pressure gradient east of the typhoon center are stronger than the FNL analysis at this time and may induce a more northward typhoon movement in the forecast. Inclusion of the DVI with the vortex center relocated to the best track position does not fully remedy the biases in the wind analysis in the vicinity of the typhoon after DA, thus, still resulting in an eastward track deviation at earlier stages as seen in Figure 17. In addition to forecast sensitivity to initial analysis errors, track predictability is also tied to the relative performances of model physics schemes that affect the forecast skill as well.