## 6 Summary and Discussion  
The technical possibility of accurately forecasting the exceedance of wind speed limits, in addition to the wind speed trend, is studied for a bridge in Hong Kong with a full set of anemometer data during two recent tropical cyclone events, namely Saola and Koinu in 2023. In both cases, accurately forecasting the timing of the maximum wind speed appears to be rather difficult, even with the use of the state-of-the-art NWP coupled CFD model simulation with a spatial resolution of down to 4 m horizontally. This is mostly attributed to the limitations of the global models in accurate forecasting the tracks and the structures  
Figure 18: (**a**) Simulated 10 min mean wind speed and direction distribution by PALM at a height of about 70 m above mean sea level and (**b**) vertical cross-section of wind speed and direction across the bridge at 17:50H, 8 October 2023 in local time. The cross-section location is depicted by the cyan line in Figure 2a, where a smaller value on the x-axis indicates a location further south. of the cyclones. Saola and Koinu are found to be rather difficult to forecast due to their rather small cyclonic circulation. There could be around a 1 to 2 h difference in the timing of the maximum wind at the bridge between the actual and the simulated data for Saola. The difference is even larger and up to 4 h in the case of Koinu. Moreover, the wind direction is not simulated well by the models. Nevertheless, the general trend of the wind speeds is reasonably reproduced by the CFD model in the Saola case as the correlations for the 10 min mean wind speed are around 0.6 or above for most of the anemometers. However, negative correlations are observed for Koinu. In terms of the critical wind speed for traffic control, the CFD simulation manages to predict the exceedance of the critical wind speed, and the timings for exceedance are, in general, around or before the actual occurrence for Saola. This suggests that CFD simulation could provide some guidance for operational traffic control if such results are available in real time. However, its practical use for Koinu is rather limited due to the large difference in the timing of the maximum winds. With the use of the CFD model, it is possible to forecast a maximum wind speed that is higher by 5 to 10 m/s over the mesoscale meteorological model for the two anemometers at the top of the bridge arc, albeit still lower than the actual data by around 10 m/s in terms of the 10 min mean wind speeds. For the other anemometers at an elevation closer to the bridge deck, which better represent the wind condition to be experienced by the vehicles on the bridge, the maximum winds predicted by the CFD model are generally similar to the observations. This opens up the technical possibility of forecasting the maximum wind speed with higher accuracy. Moreover, the CFD model manages to forecast well the uneven distribution of the wind at the eight anemometers along the bridge. Thus, the technical feasibility of forecasting the wind distribution and identifying the portions that are more vulnerable to wind hazards along bridges/railways may be explored, although the forecast accuracy is still far from being practically useful in real-time applications. Nevertheless, this might provide valuable insights during the design phases of bridges/railways. The applicability of the current approach is mainly limited by the ability of the global models to capture the track and wind structure of the tropical cyclone. Instead of using the NCEP's GFS to drive the coupled model, another simulation using the Global and Regional Assimilation Prediction System [36; 37] mesoscale model (GRAPES-Meso) to drive the coupled model was performed for the case of Saola. GRAPES-Meso has a horizontal resolution of 3 km and might better resolve the horizontal structure of a tropical cyclone. The model run initialized at 2023090100Z was used.