# Can LiDARs Replace Meteorological Masts in Wind Energy?
## 2 Test Site and Measurement Setup  
The measurements were conducted at the Fukushima Renewable Energy Institute, AIST (FREA) test site located in Koriyama city of Fukushima prefecture, Japan, about 200 km northeast of Tokyo (cf. Figure 1). Koriyama city is inland from both the Pacific Ocean and the Sea of Japan, and it is a valley surrounded by mountains and highlands. Figure 2 presents the topography around the test site,based on Shuttle Radar Topography Mission (SRTM) data [26], which has a spatial resolution of about 30 m. As shown in Figure 2a, the FREA test site is located at an altitude of about 300 m above sea level, and has a moderately complex terrain. When considering the area of 500 \(\times\) 500 m\({}^{2}\) with the test site at the center, the mean height above sea level is 291 m, and the minimum and maximum elevations are 268 m and 324 m, respectively. In order to quantify the complexity of the terrain, the Ruggedness Index (RIX), which gives the extent of steep slopes in a given area, is depicted for this region in Figure 2b. The RIX is widely used in WAsP software [27]. Following the conventional practice used with WAsP, we showed the ratios of slopes higher than 17\({}^{\circ}\) within the radius of 3.5 km and 7 km. It can be appreciated that wind directions in the range of 270\({}^{\circ}\) to 30\({}^{\circ}\) will be influenced by the local orography. Figure 3 shows the 60 m meteorological mast (met-mast) and the profiling LiDAR installed at the site. The met-mast is a triangular lattice tower with a side length of 0.6 m. In terms of the structure and height, this is a typical met-mast design used in the Japanese wind energy industry. Met-masts taller  
Figure 1: Location of the FREA test site in Koriyama. Figure 2: (**a**) Terrain complexity of the region around the test site; (**b**) Ruggedness index of the site. than 60 m have to follow stricter earthquake-related regulations, making them more expensive and less attractive among developers. Therefore, it is appropriate as a reference for performing a long-term comparison of LiDAR measurements. The met-mast is equipped with five opposing boom pairs of length 3.5 m, installed at heights between 9 m and 49 m at intervals of 10 m. The boom pairs are roughly oriented towards 20\({}^{\circ}\) and 200\({}^{\circ}\), although there is a deviation of a few degrees in the orientation at each height (cf. Table 1). Sensors are mounted on 0.71 m high vertical poles above the edge of the booms. Two sonic anemometers are installed at heights of 40 m and 57 m. Cup anemometers are installed at heights from 10 m to 50 m at intervals of 10 m, while wind vanes are installed at 10 m, 30 m and 50 m. To avoid interference due to the tower and the opposite sensors, measurement data for which the wind direction was \(\pm\)10\({}^{\circ}\) of the corresponding boom direction was discarded from the analysis. The boom length and the installation height of the sensors were made compatible with the IEC61400-12 standards [28] to mitigate influences from the tower structure. To further avoid the interference due to the tower and the opposite sensors, measured data for which the wind direction was within \(\pm\)20\({}^{\circ}\) of the corresponding boom direction were also discarded from the analysis. This was done in accordance with the requirements of the International Electrotechnical Commission (IEC) standards. The sonic anemometers collect three dimensional wind speeds at a sampling frequency of 10 Hz, whereas the cup anemometers and wind vanes collect horizontal wind speeds and directions at a sampling rate of 1 Hz. The study uses the Windcube V2 (WLS7-724) profiling LiDAR. The LiDAR operates in the DBS mode in which the LiDAR beam swings from north to east to south to west and in vertical directions.