## 4 Discussion  
The validation of the performance of very high-spatial resolution (333-m), state-of-the-art NWP model against four observation data in the region demonstrated that the model accurately predicted the strong westerly wind on 18 September 2020 in Auckland. Both gust and mean wind speed forecasts showed high statistical scores against observations. Accurate predictions of the value and time of the extreme wind event by the NWP model show a great potential in mitigating wind hazards in urban areas by developing risk forecast models and warning systems. The coupling of the NWP model with CFD simulations demonstrated that wind speeds can vary significantly in a small spatial scale due to the localised effects of structures, which are not captured by NWP models. It was shown that the bridge structure can either shelter the incoming airflow or increase the mean wind speed by up to 20% at the middle of the bridge where vehicles travel. In addition, the CFD results simulated the flow separation from the top of the bridge arch, where an anemometer is mounted. Several studies have investigated the cross-wind speed required to tip over vehicles with different sizes, e.g., [47; 48; 49]. Knowing these criteria along with the wind speed-ups and variations along the bridge could be used to develop a forecast model for the bridge, similar to the Vehicle OverTurning (VOT) Model proposed by [50]. The forecast model will be beneficial to operational meteorologists when issuing weather warnings. The model will be able to identify the occurrence of high-wind conditions and prevent accidents such as the one that happened on 18 September 2020 in Auckland, New Zealand, and the subsequent costs and disruptions. The importance of having a warning system becomes more noticeable when we consider the average reoccurrence interval of strong wind events. For instance, for the westerly wind that happened in Auckland, New Zealand, it was reported that at the time that the trucks were tipped over on the bridge, the gust speed was 35 m s\({}^{-1}\) (neglecting the effect of the flow separation from the bridge arch). It is instructive to use New Zealand's wind loading standard, AS/NZS1170.2:2011 [37] to estimate the return period of such gusts. This analysis can provide useful information on how frequently they might have occurred in the past, and whether or not it was a \"freak\" gust, as reported in the media. For wind from the westerly quarter, the upstream fetch is the water of the Waitemata Harbour and is Terrain Category 1. Using Table 4.1 in [37], we can relate this gust speed to a gust speed at a height of 10 m in Terrain Category 2, which is the reference conditions for the standard. Figure 12: Velocity iso-surface showing the locations where the velocity is 18 m s\({}^{-1}\). The top arch is removed from the visualisation to show the iso-surface on the road more clearly. The maximum height of the bridge is 64 m. If the anemometer height is taken as 64 m, then according to [37], the terrain/height multiplier is 1.26 and is not particularly sensitive to height. This means that a gust of 35.3 m s\({}^{-1}\) at the bridge corresponds to a gust of 35.3/1.26 = 28 m s\({}^{-1}\) at the standard conditions at a height of 10 m in terrain category 2. Referring to the regional wind speed for New Zealand (Table 3.1 in [37]), it is evident that a wind speed of 30 m s\({}^{-1}\) corresponds to a return period of 1 year, and so interestingly, this means that according to the wind loading standard, the gust that tipped the trucks over in September 2020 can be expected to occur more often than annually. Therefore, perhaps the fact that trucks have not been tipped over more frequently on the bridge is because a high-sided light truck has not been in the high gust speed location at the same time as it has occurred. The coupling of NWP and CFD performed in this study can be extended to other parts of Auckland, or any other city, to investigate the resilience of infrastructure against strong wind events.