## 2 Methods and Data  
The NWP model used in this study is operated by the National Institute of Water and Atmospheric Research Ltd. NIWA (The National Institute of Water and Atmospheric Research or NIWA), is a Crown Research Institute of New Zealand. Established in 1992, NIWA conducts commercial and non-commercial research across a broad range of disciplines in the environmental sciences. [[https://www.niwa.co.nz/](https://www.niwa.co.nz/)]([https://www.niwa.co.nz/](https://www.niwa.co.nz/)), accessed on 26 April 2021 and is called the Auckland Model. It is based on the UK Met Office London Model [28] and is a 333-m horizontal resolution convection-permitting configuration of the Unified Model [28; 29]. The model set up is 300 \(\times\) 300 horizontal grid-points, centred on the Auckland Harbour Bridge with a terrain-following 140-level vertical coordinate system over the lowest 40 km of the atmosphere. This increased resolution has shown significant improvements of near surface temperatures [30; 31]. The Auckland Model is one-way nested inside NIWA's 4-km resolution NZLAM (New Zealand Limited Area Model), which is data-assimilating and uses the Global Atmosphere 6 science configuration [32] with an intermediate nest at 1.5 km resolution to ease the transition from 4-km to 333-m resolution. The scientific configuration used is the Regional Atmosphere-Land 2 \"Tropical\" (RAL2-T) configuration [29]. The Auckland Model is run four times daily out to 36 h ahead. Figure 1 shows schematically the process of downscaling the 4 km NZLAM to the 1.5 km model and lastly to the 333-m model. It should be noted that the NZLAM4 domain is much larger than just over New Zealand, with its western boundary covering much of eastern Australia and eastern boundary extending past the international dateline. Figure 1 shows only part of NZLAM4 that covers New Zealand. Outputs of the 333-m model were then used to setup the inlet profiles of the CFD model. As can be seen in Figure 1, the CFD domain includes only the isolated bridge model that has most of the main structures of the bridge with the very small details removed. The computational domain extends 3 km  
Figure 1: From left to right showing the process of the downscaling the 4-km NZLAM model (full model domain not shown) to 1.5-km (green box) and to the 333-m (red box) around the Auckland region. The outputs of the 333-m model are fed to the CFD simulation with a 4-m grid resolution around the Auckland Harbour Bridge. The yellow arrow shows the approximate location of the truck overturning incident. horizontally from the centre of the bridge and 1.5 km vertically to the top of the domain. Relatively fine mesh cells were generated around the bridge with a resolution of 4 m, and even finer down to 0.02 m around the smaller details of the bridge (see Appendix A, Figure A1). The size of the cells gradually increases far from the bridge (see Appendix A, Figure A2). Tetrahedral cells are used to ensure the complex geometry of the bridge is resolved. The total number of cells is about 32.7 million. In order to validate the forecasts of the 333-m NWP model, observation data from four stations across the Auckland region were used. Figure 2 shows the locations of the stations (yellow pins) as well as the Auckland Harbour location (red pin). Table 1 summarises the name of the stations used along with their coordinates and wind speed measurement heights above the ground. In the case of the Sky Tower station, there are two anemometers mounted close to the top of the tower at a height of 318 m, with one facing northwest (NW) and the other southeast (SE). Data from both the anemometers are compared with the NWP model at the location of the Sky Tower. For the event on 18 September 2020, mean and gust wind speed time series (Figure 3) were extracted from the relevant forecast cycle of the 333-m NWP model. Figure 3a shows the model forecasts of the mean wind speeds at different vertical levels upstream of the Auckland Harbour (red pin in Figure 2).