## 2 Actual Observations  
There are eight anemometers along the bridge. Their locations with a scale bar can be found in Figure 2a, where CB1, CB2, CB3, CB4, CB5, CB6, CB7, and CB8 are approximately 30, 40, 20, 20, 70, 70, 20, and 20 m above the mean sea level, respectively. A vertical sectional view of part of the bridge is shown in Figure 2b for reference. The anemometers CB5 and CB6 are at the top of the bridge arc, while the anemometers CB1, CB3, CB4, CB7, and CB8 are closer to the bridge deck, and they might better represent the winds experienced by vehicles, cyclists, or passengers. Taking CB1 as a reference point, the horizontal distances of CB2, CB3, CB4, CB5, CB6, and CB7 from CB1 are about 210, 370, 370, 420, 420, 490, and 490 m, respectively. In the following discussions, for simplicity of presentation, only the time series of the wind data of CB1, CB3, CB5, CB6, CB7, and CB8 are given.  
Figure 3 shows the measured 10 min mean wind speeds for the eight anemometers in the case of Saola at (a) around the time when the simulated wind speeds were the highest (for comparison with the simulation result later) and (b) the time when the measured wind speeds at CB5 and CB6 were the highest. Regarding Figure 3a, the wind speeds for anemometers CB5 and CB6 were unfortunately unavailable as they did not pass the quality check. The wind speeds at the other six anemometers were all below 20 m/s. As shown in Figure 3b, the wind speeds at anemometers CB5 and CB6 were the highest, reaching around 40 m/s. The wind speeds at the other six anemometers slightly increased, mostly in the range of 20 to 28 m/s.  
Figure 1: The surface isobaric charts at (**a**) 08H, 1 September 2023 and (**b**) 08H, 8 October 2023. Chinese translations are shown in the figure. True color satellite image of Himawari-9 satellite of Japan Meteorological Agency at (**c**) 08H, 1 September 2023 and (**d**) 08H, 8 October 2023. All times are local time (+8 UTC).  
Figure 4 shows the measured 10 min mean wind speeds for the eight anemometers in the case of Koinu at (a) around the time when the simulated wind speeds were the highest and (b) the time when the measured wind speed at CB5 was the highest. As shown in Figure 4a, the anemometers CB5 and CB6 recorded wind speeds of around 15 m/s, while the other six anemometers recorded wind speeds of around 10 m/s only. According to Figure 4b, the anemometer CB5 had the highest wind speed of around 26 m/s. The anemometers CB1, CB2, CB3, CB6, and CB7 had wind speeds ranging between 10 and 16 m/s. The wind speeds from the anemometers appear to be rather uneven.  
Figure 2: (a) Locations of eight anemometers along the bridge under study. The cyan line represents a cross section perpendicular to the bridge where the simulation winds on it will be shown in Figures 11b and 18b. (b) Vertical sectional view of the bridge projected onto the horizontal red line in (a).  
Figure 4: (**a**) The 10 min mean wind speeds and directions for the eight anemometers at 17:50H, 8 October 2023 in local time and (**b**) at 22:25H, 8 October 2023 in local time.  
Figure 3: (**a**) The 10 min mean wind speeds and directions for the eight anemometers at 20:10H, 1 September 2023 in local time and (**b**) at 21:30H, 1 September 2023 in local time.