## 4 Results and Discussion  
### Overall Meteorological Characteristics  
In Table 7, the annual mean meteorological parameters observed for the three cases are provided where _U1_ and _U2_ are the average wind speeds at the heights of _h1_ and _h2_, _U3_ is the average wind speed at the hub height of _h3_, _T1_ and _T2_ are the mean temperatures at the heights of _h1_ and _h2_. Besides, _\(\alpha_{h1-h2}\)_ and _\(\alpha_{h2-h3}\)_ are the mean wind shear exponents calculated with velocities at _h1_ and _h2_, and _h2_ and _h3_ respectively.  
The results indicate the difference of the wind speed distribution between the plain and mountain areas. In the HNH case, where the terrain is flat, the inconspicuous mixing of atmospheric turbulence and the weak exchange of momentum between the vertical layers result in a vertical gradient of wind speed and a large wind shear exponent. Because of the acceleration effect on the wind speed on the mountaintop, the wind shear exponents in GFC and SJB are smaller than the one in HNH.  
For the SJB plateau complex terrain, at 30 m (Figure 2a) and 70 m (Figure 2b), the south direction is the most frequent wind direction with the occurrence percentages of 15.81% and 14.50%, respectively. The changes of wind speed and direction in this area are limited, because at 30 m, the wind direction is less affected by the terrain, and the height difference between the two measurement levels is only 40 m. At the same time, under the complex terrain condition, the mixing of atmospheric turbulence, the momentum exchange in the vertical direction are strong, resulting in a small wind shear exponent.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Number** & **Site Name** & _h1_ **(m)** & _h2_ **(m)** & _h3_ **(m)** & **Time Interval (min)** & **Data Collection Period** \\ \hline
1 & SJB & 30 & 50 & 70 & 10 & 27 August 2015–26 August 2016 \\
2 & GFC & 30 & 50 & 80 & 10 & 1 August 2012–31 July 2013 \\
3 & HNH & 10 & 60 & 100 & 10 & 15 September 2014–16 September 2015 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Summary of measurement parameters.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Site Name** & _U1_ **(m/s)** & _U2_ **(m/s)** & _U3_ **(m/s)** & _T1_ **(\({}^{\circ}\)C)** & _T2_ **(\({}^{\circ}\)C)** & \(\alpha_{h1-h2}\)** \\ \hline SJB & 5.34 & 5.46 & 5.71 & 8.27 & 8.06 & 0.0435 \\ GFC & 5.07 & 5.10 & 5.17 & - & - & 0.0115 \\ HNH & 2.73 & 4.61 & 5.44 & 14.30 & - & 0.2924 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Wind data information.  
(Figure 2d), the SE direction is the most frequent direction (32.17%), and ESE (17.63%) is the secondary predominant direction. At the same time, the overall wind speed interval is dominated by a low wind speed interval of 3-8 m/s. This is because the complex mountain terrain has a great impact on the wind direction. At the same time, the topographic factors also cause strong mixing in vertical, which results in smaller values of wind shear exponent, so there is no significant change in the wind speed.  
For the plain area HNH, at 10 m (Figure 2e), the S direction occurs most frequently (20.75%), and at 80 m (Figure 2f), it is also the S direction (22.90%). The wind direction at 10 m and 100 m are similar, but the wind speed at 100 m is significantly higher. This is related to the flat terrain condition of this area. The atmosphere is inadequately mixed in the vertical direction in this terrain condition, which leads to a larger wind shear exponent and causes a significant change in the vertical wind speed.