## 2 Methods and Data
Since the RANS approach is used in this study, which only provides time-averaged flow properties, the mean wind speed outputs of the 333-m model were used to define the inlet profiles for the CFD simulation, as shown in Figure 4. In addition, Figure 3b shows the instantaneous and maximum gust wind speeds over a 10-min period at the location of the Auckland Harbour Bridge during the event. Figure 2: Locations of the measurement sites (yellow pins) used for the validation of the NWP outputs. The red pin shows the location of the Auckland Harbour Bridge. The aerial image is from Google Earth. For the CFD simulation, a pressure-driven approach [33] was used to simulate the atmospheric boundary layer (ABL), utilising a three-dimensional, steady RANS simulation using the \(k-\omega\) Shear Stress Transport (SST) [34] turbulence model. The mean wind speed forecasts of the event shown in Figure 3a were used to fit the velocity profile (Equation (1), Figure 4a) and calculate the friction velocity (\(u_{*}\)). It is shown in Figure 3a that the forecast mean speeds, and the fitted profile (Figure 4a), range from 11 m s\({}^{-1}\) to 18 m s\({}^{-1}\) at heights of 1 m and 103.7 m above the ground, respectively. Then, the corresponding profiles of the turbulent kinetic energy (\(k\)) and turbulent eddy frequency (\(\omega\)) (Equations (2) and (3) and Figure 4b,c, respectively) were calculated. \(U,k\), and \(\omega\) profiles were set as the inlet boundary conditions for the computational domain. The symmetry boundary condition is used at the top boundary, the pressure-outlet boundary condition with zero-gauge pressure applied at the outlet of the domain, and for the ground, the automatic near-wall treatment [34; 35; 36] was applied, replicating the sea surface roughness (\(z_{0}\)) with a roughness length of 0.002 m [37]. The blending function employed in the SST model allows the use of the attractive near-wall performance [35] of the \(k-\omega\) model [38] in the near-wall \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Station Name** & **Latitude** & **Longitude** &
\begin{tabular}{c} **Height above the** \\ **Ground** \\ \end{tabular} \\ \hline Auckland Aero & \(-37.00813\) & \(174.78873\) & \(10\) m \\ MOTAT EWS & \(-36.86297\) & \(174.71185\) & \(10\) m \\ Whenuapai & \(-36.79300\) & \(174.62400\) & \(10\) m \\ Sky Tower (SE anemometer) & \(-36.85004\) & \(174.76242\) & \(318\) m \\ Sky Tower (NW anemometer) & \(-36.85004\) & \(174.76242\) & \(318\) m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of the measurement stations used for validation along with their coordinates and heights above the ground surface. Figure 3: Mean (**a**) and gust (**b**) wind speed time series outputs of the 333 m model upstream of the Auckland Harbour Bridge, during the extreme westerly event on 18 September 2020.  
region, and for the rest of the flow, the \(k-\varepsilon\) model [39] is utilised. The automatic near-wall treatment switches automatically between wall functions and a low-Reynolds number formulation based on the grid spacing [35; 40]. A detailed description and formulation of SST and the automatic near-wall treatment can be found in [35; 40]. The Auckland Harbour Bridge runs effectively south-north and the wind was westerly for the event on 18 September 2020, meaning that the approaching airflow to the bridge is only over water and not over land. Therefore, the isolated bridge setup for the CFD simulation and the sea surface roughness assumption are sensible.