## 2 Setup of the Wind Tunnel Test  
The wind tunnel testing method was utilized to study the characteristics of the wind field involved with a hilly terrain, and the tests were completed in ZJU-1 wind tunnel in Zhejiang University, China. The wind tunnel has a testing section of 4.0 m wide and 3.0 m high, and the testing wind speed ranges from 0.0 to 50.0 m/s. However, the shapes of the hills in nature are diverse and the profiles of the corresponding wind fields are different. For design, it is necessary to survey the wind characteristics information by using a generalized hill shape and a standard flow field. Therefore, a cosine-squared cross-section (Figure 1) is adopted herein for the wind tunnel test, and can be expressed as:  
\[z_{s}(x,y)=\left\{ \begin{array}{ll}H\cos^{2}(\pi\frac{\sqrt{x^{2}+y^{2}}}{2 L}),&\sqrt{x^{2}+y^{2}}<L\\ 0,&\sqrt{x^{2}+y^{2}}\geq L\end{array} \right. \tag{1}\]  
In Equation (1), the hill height \(H\) is 100 m and the base radius \(L\) is 350 m. The streamwise, spanwise and vertical directions are denoted by \(x,y,\) and \(z\), respectively, as shown in Figure 1. The origin is set at the center of the bottom of the hill. A local vertical coordinate \(Z_{i}=z-z_{s}\) is employed to describe the height above the surface of the hill, where \(z_{s}\) is the height of the points on the hill surface. The scale of the model is 1:1000. Thus, the base radius of the model for test is 0.35 m, and the height of the model is 0.1 m. The model of the hill is shown in Figure 2.  
Figure 1: Coordinate system and notations. Remarkable effects of the surface roughness on the characteristics of the wind fields over the hilly terrain have been emphasized by previous studies [23; 24; 25]. With consideration of that the vegetation on the surface of the real hill may create a certain surface roughness, the average height of the vegetation is assumed to be 2.0 m. To simulate this vegetation-induced surface roughness, plastic fibers with a height of 2.0 mm were distributed over the surface of the hill model. The Cobra Probe [26] was used to measure the wind speeds in the hilly terrain-disturbed ABL wind. The sampling frequency is 1250 Hz, and the duration is approximately 60 s for each measuring. The oncoming wind field belongs to Type A (a reference to offshore seas and islands, coasts, lake shores and desert areas) according to GB 50009-2012 [27]. The oncoming neutral ABL wind is physically simulated by using a passive method with a scale ratio of 1:1000, namely, spires in association with floor roughness arrays are employed for the simulation. The simulated wind speed \(V\) normalized by \(V_{H}\) (the wind speed at a height of gradient wind) at the reference height, along with the streamwise turbulence intensity \(I_{u}\) is shown in Figure 3, as depicted in the full scale dimension. The wind speed and turbulence intensity for Type A [27] can be expressed as:  
\[V =V_{10}(z/10)^{0.12} \tag{2}\] \[I_{u} =I_{10}(z/10)^{-0.12} \tag{3}\]  
where \(V_{10}\) and \(I_{10}\) are the mean wind speed and turbulence intensity, respectively, at the height of 10 m. The measured profiles of mean wind speed and turbulence intensity are close to the requirements in GB 50009-2012. The streamwise wind power spectra attained at the height of 0.09 m and 0.15 m in the wind tunnel test are generally close to the von Karman spectrum [28], as shown in Figure 4a,b, in which \(n\), \(S\), and \(\sigma\) are the frequency, spectrum and standard deviation of the fluctuating wind speed, respectively. Overall, the simulated oncoming wind fields are generally in accordance with the requirement of the code [27]. Figure 2: The hilly terrain model. Twenty one test points with a sequential number assigned were merely placed at the half of the hill, due to the symmetry. Furthermore, the test points are arranged along the five lines above the hill surface, as show in Figure 5. Thus, for analysis, the experimental results measured are mirrored with respect to x-z plane.