## 3 Results  
### Comparative Analysis of Horizontal Wind Speed Profiles from 50 to 1500 m  
Figure 2 shows the applicability of ERA5 horizontal wind speeds at various heights. Specifically, the horizontal wind speed of the ERA5 reanalysis datasets at a height of 50 m corresponds well to the observed value (Figure 2(a1)), but there remains a certain error, with a root mean square error (RMSE) of 2.51 m/s. The effect of local circulation possibly contributes to this phenomenon. The complex flow field at the top of the mountain changes from time to time. Therefore, it is not easy for the ERA5 to capture the phenomenon of an abrupt change of the wind speed very well. At the observation site, the terrain is complex, and there are a number of circulation currents that affect the local horizontal wind speeds. The site is surrounded by mountains, and there is a noticeable valley wind at the observation point. In terms of the specific weather events, significant deviations in the ERA5 reanalysis data occur; for example, during the stormy weather on 21 June,  
the behaviour of the horizontal wind of ERA5 data is relatively low. Another extreme weather event is the typhoon on 28 July, and the ERA5 values are much higher than the observed values. It is also interesting to find that, due to the topography of the mountain, the occurrences of valley winds make the observed horizontal wind speeds higher than the ERA5 values. The horizontal wind speeds at 100 m are compared in Figure 2(b1). The observed winds are stronger than the ERA5 values due to the rapid changes in 13-14 May. On 25 May, there was light rain in the vicinity of the mountain, and a drizzle in the area not far away from the adjacent Wuyi county. The ERA5 reanalysis data could not resolve the weather process within such a small scale. Figure 2(c1) gives the inter-comparison of the 200 m horizontal wind speed, and we find there is convective weather on 8 May, resulting in the underestimate of the ERA5 reanalysis.  
The duration of the observations in this paper was in the rainy season, which is often characterized by strong convective weather. This abnormal weather requires fine resolutions and intensive observations to resolve, which entails a need for further improvement in the ERA5 reanalysis. In particular, a typhoon that was originally forecast to pass through the area from 24 to 30 July travelled in a path adjacent to the site, during which the ERA5 wind speed is higher than the observations. In Figure 2(d1), the agreement between the  
Figure 2: Time series (**a1–f1**) and scatter plots (**a2–f2**) of the observed horizontal wind speeds at various heights compared with the ERA5 reanalysis data values. The zoomed-in time series of 2 and 3 June 2023 were selected for comparison, with the red and blue lines representing the horizontal wind speeds from ERA5 and Doppler wind lidar, respectively. The areas circled by ellipses are the locations of wind speed changes during extreme weather and heavy rainfalls.  
ERA5 and the measurements is very good at 500 m above the mountain top, but there are still some fast variations in the actual wind speeds that cause some differences. The subplot of Figure 2(e1) shows that the observed horizontal wind speed at 1000 m is higher than the ERA5 reanalysis data, which may be attributed to the systematic errors of the ERA5 reanalysis data. Although the Doppler lidar is undetectable to a height of 1500 m most of the time because of the frequent fogs and clouds (Figure 2(f1)), the available observations obtained show that the horizontal wind speed of ERA5 is basically smaller than the measurements. The deviations can be clearly observed in the fitted curve in Figure 2(f2).