## 3 Results
### Time Series Analysis of Wind Speed at Different Heights
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Height** & **Maximum** & **Mean** & **SD** & **Range** & **IQR** & **Skewness** & **Kurtosis** \\ \hline
8 m & 11.2 & 1.463 & 0.950 & 11.1 & 0.8 & 2.015 & 5.445 \\
15 m & 11.5 & 1.762 & 1.106 & 11.4 & 1.1 & 2.021 & 5.254 \\
32 m & 13.7 & 2.252 & 1.350 & 13.5 & 1.3 & 1.881 & 4.527 \\
47 m & 16.1 & 2.567 & 1  
From Table 3, it can be observed that the average wind speed increases with height, which can be attributed to the decreasing influence of surface friction on wind speed as the height increases. Simultaneously, with increasing height, the standard deviation and range of wind speed also increase, indicating a greater variability and uncertainty in wind speed in higher-altitude regions, implying an increased complexity in the influences of various atmospheric processes on high-altitude wind fields. The gradual decrease in skewness and the decline in kurtosis reflect a transition in wind speed distribution from a distinct positive skewness to a more symmetric shape from low to high speeds. This transition in distribution shape may be related to the dynamic characteristics of atmospheric stratification and vertical airflow, where the diffusion of wind speed extremes diminishes at higher altitudes, leading to an overall trend towards a normal distribution. When conducting an overall trend analysis using monthly average wind speeds, we observed significant seasonal characteristics in the annual changes in wind speeds. However, these seasonal characteristics are cyclical, with wind speeds experiencing significant fluctuations during different periods of the year. Using annual averages does not effectively quantify the trends in wind speed changes at the quarterly level. To better investigate the variation in wind speed at different altitudes in different seasons, we classified the wind speed into four seasons, namely, spring, summer, autumn, and winter, as shown in Figure 5.  
The least squares analysis of linear regression models over the past three decades reveals a predominantly downward trend in wind speeds across most seasons and altitudes. Notably, in spring, the decline is particularly pronounced at the lower and middle levels, with model slopes of 0.0258 and 0.0238, respectively, indicating significant velocity decreases. In contrast, the upper level exhibits a milder reduction with a slope of 0.0107 (Figure 5a). During summer, wind speed fluctuations are relatively stable but still show an overall decline (Figure 5b). Autumn presents the most substantial decrease at the middle level, with a slope of 0.0274, while both the lower and upper levels also show noticeable  
Figure 5: Changes in average wind speed at different altitude levels in different seasons. (**a**) Average wind speed in spring. (**b**) Average wind speed in summer. (**c**) Average wind speed in autumn. (**d**) Average wind speed in winter. declines, with slopes of 0.0169 and 0.0160, respectively, confirming a uniform reduction across all levels (Figure 5c). Winter, however, sees a slight increase in wind speed at the upper level, with a slope of 0.0025, whereas the lower and middle levels continue a mild downward trend, with slopes of 0.0078 and 0.0143, respectively (Figure 5d). Collectively, these patterns highlight that reductions in wind speeds are most significant in spring and autumn at the lower and middle levels, while summer exhibits relatively stable yet lower wind speeds. Integrating these data with monthly trends, it becomes evident that despite annual increases in spring and winter, there is a long-term downward trend in wind speeds across all seasons, including summer. The slight winter increase at the upper level may be attributed to inherently higher wind speeds and greater variability at this altitude. Table 4 details the fitting functions for different height levels across seasons. To study the trend of wind speed changes among various altitudes, we use Power Law Wind Profile Theory to describe the relationship between different altitude levels.