## 3 Results
### CFD Results  
Velocity streamlines for the westerly wind on 18 September 2020 passing through the Auckland Harbour Bridge are shown in Figure 7a, depicting wind speed-ups on the edge of the road as well as between the struts of the bridge. Figure 7b depicts the mean speed on a plane at the centre of the bridge along the road. The locations of high (red) and low (blue) wind speeds can be seen. The sheltering effects of the bridge components result in lower wind speeds. To better realise the flow pattern across the width of the bridge, the results are presented on two cross-sectional planes (Figure 8). Figure 9a,c and Figure 9b,d show the mean wind speed and wind speed-ups on these planes, respectively. The wind speed-up is defined as the wind speed at a specific location and elevation divided by the undisturbed upstream wind speed at the same elevation. The location of Plane 1 is chosen such that it goes in-between the side struts of the bridge, while Plane 2 goes through a vertical beam. The planes are selected to illustrate how the area reduction due to the presence of the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Max Gust Wind Speed**} & \multicolumn{3}{c}{**Mean Wind Speed**} \\ \hline
**Station Name** & **Correlation** & **Mean Bias** & **RMSE** & **Correlation** & **Mean Bias** & **RMSE** \\ \hline Auckland Aero & 0.87 & 1.32 & 2.22 & 0.80 & \(-2.08\) & 2.39 \\ Whenuapai & 0.93 & \(-1.14\) & 2.15 & 0.85 & \(-1.60\) & 1.97 \\ MOTAT & 0.81 & 1.24 & 2.91 & 0.76 & 0.62 & 1.45 \\ Sky Tower (SE anemometer) & 0.47 & \(-3.73\) & 5.44 & 0.15 & 2.57 & 5.14 \\ Sky Tower (NW anemometer) & 0.65 & \(-4.67\) & 5.42 & 0.49 & 1.03 & 2.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary statistics for comparison between the observed and Auckland Model forecast gust and mean wind speeds at the location of the four stations. Figure 7: Overall airflow behaviour around the harbour for the westerly wind: (**a**) velocity streamlines; (**b**) contour of mean speed at the centre of the harbour. bridge structure (Plane 1) results in wind speed-up (Figure 9a). On the other hand, wind speeds on Plane 2 depict the sheltering effect behind the vertical beam (Figure 9b). It can be seen in Figure 9b that the mean speed on Plane 1 can be increased by up to 1.15-1.20 in the middle of the bridge where vehicles travel. This speed-up is due the sudden reduction in the area between the struts. In contrast, on Plane 2 (Figure 9d), the  
Figure 8: Two cross-sectional planes selected to present the wind speed results. Figure 9: Wind speed results on two cross-sectional planes along the width of the bridge: (**a**) mean speed on Plane 1; (**b**) mean speed-up on Plane 1; (**c**) mean speed on Plane 2; (**d**) mean speed-up on Plane 2.  
wind speed-up is considerably less than 1.0 (blue region in the middle of the bridge), which is because of the sheltering effect of the structure. This illustrates the significant variability of wind speed along the bridge. It is also noteworthy that the wake created by the bridge extends far downstream of the bridge. The NWP gust wind speed timeseries presented in Figure 3b was gust wind speed at 10 m above the ground. The maximum height of the Auckland Harbour Bridge is 64 m. Therefore, to estimate the gust wind speed at the top of the bridge, the values in Figure 3b are multiplied by a height/terrain factor obtained from Table 4.1 of [37]. The NWP gust speed timeseries at 10-m and 64-m heights are compared in Figure 10. It can be seen that the maximum gust wind speed during this event at 64-m is 25 m s\({}^{-1}\). In Figure 3a, it was shown that at a height of 103.7 m, the mean forecast speed was 18 m s\({}^{-1}\). The bridge has a clearance height of about 43 m (water to roadway). Figure 12 illustrates velocity iso-surfaces, showing locations where the wind speed is 18 m s\({}^{-1}\). It is evident that the structure of the bridge affects the upstream airflow such that a mean wind speed of 18 m s\({}^{-1}\) is predicted on the roadway of the bridge.