## 1 Introduction  
Wind characteristics over mountainous valleys are critical to many wind-related issues, such as wind actions of long-span bridges and transmission lines, wind farm site selection, prediction of pollutant dispersion, and so on. In particular, wind characteristics at mountainous valley bridge sites such as design wind speed, wind yaw angles, wind attack angles, and turbulence spectra play a critical role in wind-resistant design of long-span bridges. The wind characteristics over complex mountainous terrain are significantly different from those of open areas. Therefore, reasonable determination of wind parameters at the bridge sites over mountainous terrain is important to the balance between wind-resistance safety and the economy of long-span bridges. However, the current wind-resistant design codes for bridges generally make simple corrections for mountain wind fields based on wind characteristics of flat terrain ([PERSON] et al., [1]). Therefore, it is especially necessary to investigate wind characteristics over complex mountainous terrain precisely. The main methods for studying the wind characteristics over complex mountainous terrain bridge sites include theoretical study, field measurement, experimental study, and numerical study. Theoretical studies were used to predict the wind speed-up effects over simple topographic features with slopes low enough to avoid flow separation ([PERSON] and [PERSON] [2]; [PERSON] et al. [3]), which cannot be adopted to study the wind characteristics over complex mountainous terrains. Field measurement is regarded as the most reliable approach to investigate the wind characteristics in the ABL ([PERSON], et al. [4]), which can accurately measure the wind speed and direction data at different measurement points. During the past two decades, numerous field measurements of wind characteristics over mountainous valley were conducted ([PERSON] [5]; [PERSON] et al. [6]; [PERSON] et al. [7]; [PERSON] et al. [8]; [PERSON] et al. [9]; [PERSON] et al. [10]; [PERSON] et al. [11]; [PERSON] et al. [12]; [PERSON] et al. [13]; [PERSON] et al. [14]; [PERSON] et al. [15]). However, field measurements of wind characteristics over mountainous bridge site are usually expensive, time-consuming, and can only provide wind data at a limited number of measurement points, which are easily affected by the terrain. Experimental studies of wind characteristics over complex mountainous terrain were conducted by many researchers over the last three decades ([PERSON] [16], [PERSON] et al. [1], and [PERSON] et al. [17]; [PERSON] et al. [18]; [PERSON] et al. [19]; [PERSON] et al. [20]; [PERSON] et al. [21]; [PERSON] et al. [22]; [PERSON] et al. [23]; [PERSON] et al. [24]). [PERSON] [25] pointed out that the geometric scales of complex terrain model should not be less than 1:2500-5000, and the Reynolds number should be larger than \(\text{Re}_{\text{h}}=10^{5}\) (h is the model hill height) for wind tunnel tests. However, for engineering structures located at complex mountainous terrain, such as long-span bridges, power transmission towers, and wind turbines, it is especially important, and an urgent matter, to carry out numerical simulation of wind characteristics since complex terrain makes it difficult for scholars to propose a common wind characteristic model. When the wind tunnel test model or numerical simulation model are adopted to study wind characteristics of mountainous terrain, it is inevitable to take a certain range of areas from mountainous terrain, causing \"artificial cliffs\" (shown in Figure 1) on the edge of terrain models. These artificial cliffs can deviate practical wind characteristics at the inlet boundary from these of theoretical ones, such as wind speed, wind attack angles, and turbulence characteristics. To cope with the negative influence of an artificial cliff on wind tunnel tests, some scholars have put forward with different solutions. [PERSON] et al. [26] and [PERSON] et al.