# Analysis of the Near-Ground Wind Field Characteristics during Typhoon Soulik
## 4 Quantitative Analysis of Fluctuating Wind Properties  
### Variations in the Intensity of Turbulence  
The turbulence intensity represents an indicator that quantifies the characteristic of turbulence and serves as a crucial metric for assessing wind loads in structural engineering. It is defined as the ratio of the standard deviation of component velocities to the mean wind velocity during a certain period of time. The expression is  
\[I_{i}=\frac{\sigma_{i}}{U}(i=u,v,w) \tag{12}\]  
where \(I_{i}(i=u,v,w)\) represents the turbulence intensity for every orientation and \(\sigma_{i}\) denotes the RMS of the turbulence components. Figure 5 shows the varying trends in turbulence intensity in each direction and the average wind speed through a 10 min period at heights of 15, 27, 53, 67, and 82 m. From the figure, the turbulence intensity of \(u\), \(v\), and \(w\) orientations exhibits a strong association with variations in average wind velocity. Furthermore, as the average wind speed drops below 12 m/s, the turbulence intensity diminishes in \(u\), \(v\), and \(w\) orientations. Conversely, as it exceeds 12 m/s, the turbulence intensity remains relatively constant. Figure 4: Correlation coefficients and normalized standard deviation distribution Taylor plots of each standard and measured wind profiles. Figure 5: _Cont._  
Table 1 displays the recorded ratio of turbulence intensity in each direction at different elevations, together with the outcomes of other research works. The study revealed that the turbulence intensity ratios of Typhoon Soulik in the transverse and longitudinal directions were both over one, which is inconsistent with the findings of [PERSON] et al. (2018), [PERSON] et al. (2018), and [PERSON] et al. (2012). This may be attributed to the study being situated in a valley terrain and in the peripheral region of the typhoon. Meanwhile, the ratio of vertical to longitudinal turbulence intensity falls within the range of 0.66-0.86, which aligns with the findings of [PERSON] et al. (2012). Based on the actual measurements of turbulence at various heights, box plots are drawn; they show the variation in longitudinal, transverse, and vertical turbulence intensity with height, as shown in Figure 6. From the figure, it can be seen that there is a gradual decrease in turbulence intensity as height increases. However, this decline is less significant compared to the variation in turbulence intensity with average wind velocity. Additionally, the comparison is as follows: transverse turbulence intensity > longitudinal turbulence intensity > vertical turbulence intensity. This could be due to the influence of valley topography, causing airflow to become non-uniform, thereby intensifying transverse turbulence. Frequency spectrum analysis of longitudinal and transverse wind components is conducted using Fourier transform, as shown in Figure 7. From the figure, it can be observed that the transverse wind spectrum at heights of 15, 53, and 82 m is higher than the longitudinal wind spectrum, indicating that the transmission tower is more susceptible to transverse wind effects, which may lead to an increase in transverse vibration of the structure. Therefore, in future structural designs, it is advisable to strengthen protection against transverse wind loads. \begin{table}
\begin{tabular}{c c c c c} \hline
**Researcher** & **Typhoon** & **Height (m)** & \(\mathbf{I_{u}}\)**-\(\mathbf{I_{v}}\)**-\(\mathbf{I_{w}}\)** & **Location** \\ \hline [PERSON] et al. (2018) & [PERSON] & 10 & 1:0.83:0.56 & Japan \\ [PERSON] et al. (2018) & [PERSON] & 40 & 1:0.9:0.50 & Shanghai, China \\ [PERSON] et al. (2012) & [PERSON] & 32 & 1:0.83:0.56 & Fujian, China \\  & & 15 & 1:1:28:0.66 & \\ Present Results & [PERSON] & 53 & 1:1.31:0.84 & Fujian, China \\  & & 82 & 1:1.17:0.86 & \\ \hline \end{tabular}
\end{table} 
Table 1: Turbulence intensity ratio.