## 3 Results  
### Time Series Analysis of Wind Speed at Different Heights  
In the comprehensive analysis of wind speed data from 1991 to 2020, considering the diverse trends in wind speed changes across different periods, the timeline was divided into three distinct intervals: 1991 to 2000, 2000 to 2010, and 2010 to 2020. Additionally, recognizing the influence of varying altitudes on wind speed variations, these were classified into three primary strata: lower level (8 m to 47 m), middle level (65 m to 140 m), and upper level (160 m to 320 m). During the 1991 to 2000 interval, the average wind speed at the lower level saw a decrease of approximately 0.18 m/s, while the wind speeds at the middle and upper levels increased from 3.56 m/s and 4.45 m/s to 3.62 m/s and 5.01 m/s, respectively, showing growth rates of about 1.69% and 12.58%, as shown in Figure 4a. Figure 4: Monthly average wind speed trends at various altitudes from 1991 to 2020. (**a**) Monthly average wind speed changes from 1991 to 2000. (**b**) Monthly average wind speed changes from 2000 to 2010. (**c**) Monthly average wind speed changes from 2010 to 2020. Over the decade from 2000 to 2010, a decrease in wind speeds was noted across all altitude strata. At the lower level, the wind speed fell from 2.05 m/s in 2000 to 1.87 m/s in 2010, a reduction of about 8.78%. For the middle level, the wind speed decreased from 3.62 m/s in 2000 to 3.29 m/s in 2010, roughly a 9.12% decline. At the upper level, the wind speed dropped from 5.01 m/s in 2000 to 4.73 m/s in 2010, a decrease of approximately 5.99%. Contrary to the previous decade, this period observed a declining trend in wind speeds at the middle and upper levels, as shown in Figure 4b. Between 2010 and 2020, the wind speeds at all levels remained largely stable, with only slight variations. At the lower level, the wind speed marginally decreased from 1.87 m/s in 2010 to 1.86 m/s in 2020, a decline of about 0.53%. At the middle level, there was a minor increase in the wind speed from 3.29 m/s in 2010 to 3.31 m/s in 2020, an increase of approximately 0.61%. At the upper level, the wind speed slightly fell from 4.73 m/s in 2010 to 4.67 m/s in 2020, a decrease of around 1.27%. Similarly, the consistent seasonal pattern was observed across all strata, as shown in Figure 4c. Overall, throughout the entire 30 years, the wind speed at the lower level showed a declining trend, with an overall average decrease of 16.59%, with the most significant reduction occurring in the first two decades. The primary factors for this decline likely include an increase in surface roughness due to urbanization. The wind speeds at the middle and upper levels experienced a decline after 1995 but saw a resurgence in 2015. Overall, the wind speed at the middle level decreased by about 7.02%, while at the upper level, it inversely increased by about 4.94%. These changes suggest that although the wind speeds at the middle and upper levels are affected by the urban substrate, this influence is relatively weak, and they are more likely influenced by factors related to global climate change. Table 2 shows the average wind speeds at different altitude levels for each year from 1991 to 2020. Table 3 shows the statistical measures of wind speed samples at various heights. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Y\({}_{\text{n}}\)** & **L (m/s)** & **M (m/s)** & **U (m/s)** \\ \hline
1991 & 2.23 & 3.56 & 4.45 \\
1995 & 2.36 & 3.98 & 5.21 \\
2000 & 2.05 & 3.62 & 5.01 \\
2005 & 1.95 & 3.46 & 5.02 \\
2010 & 1.87 & 3.29 & 4.73 \\
2015 & 1.85 & 3.23 & 4.60 \\
2020 & 1.86 & 3.31 & 4.67 \\ \hline \hline \end{tabular} \(\text{Y}_{\text{n}}\)—number of years; L—average wind speeds at the lower level; M—average wind speeds at the middle level; U—average wind speeds at the upper level.  
\end{table} 
Table 2: Average wind speeds at different altitude levels from 1991 to 2020.