# Coastal Wind Measurements Using a Single Scanning LiDAR
## 3 Results
### Validation of 10-Minute Mean Wind Speeds and Wind Directions
#### 3.2.3 Impact of Wind Direction on Wind Speed Accuracy  
Previous studies [19] indicated that the accuracy of wind speeds obtained from a single scanning LiDAR were significantly affected by the wind direction. The accuracy of the wind speeds with an across-wind direction, defined as perpendicular to the LiDAR heading angle \(\pm 45^{\circ}\), might be lower than those with an along-wind direction, defined as parallel to the LiDAR heading angle \(\pm 45^{\circ}\)(cf. Figure 3 for the definitions). In order to investigate the impact of wind direction on the accuracy of the retrieved horizontal wind speeds, an experiment with the same setting as Case 2 but covering a longer period of time (62 days) was conducted and is defined as Case 8 in Table 2.  
of 0.24 m/s (4.0%), and a determination coefficient of 0.992; the 100s wind directions have a mean deviation of \(-0.7\)deg, a standard deviation of deviation of 3.8 deg, and a determination coefficient of 0.998. To identify the impact of wind direction, Figure 12 shows two scatter plots for wind speed--one for along direction cases (as defined above) and one for across direction cases, based on the wind direction obtained from V1. As shown here, the results for the along-wind direction exhibit slightly better results in the standard deviation of the deviation, but no significant differences were observed between the two cases.  
To investigate the influence of the wind direction in greater detail, the ratios of the V1 wind speeds and the 100s wind speeds were plotted as a function of the V1 wind direction at a height of 47 m (Figure 13). The bin-averaged values with a 5 deg bin width and the associated 1 standard deviation bounds were also plotted. The along and across direction classifications were indicated by the dark solid and dashed horizontal lines. The blue and red arrows, respectively, indicate wind blowing from the sea to the land and wind blowing from the land to the sea. As can be seen here, most of the bin-averaged values were plotted for directions between 335 deg and 145 deg, with the winds coming from the sea sectors, had a ratio close to 1.0. On the other hand, the bin-averaged ratios for directions between 155 deg and 325 deg, with the winds coming from the land sectors, were more widely scattered. The ratios between 175 deg and 230 deg were greatly scattered due to the influence from the nearby wind turbines. Thus, the accuracy of the wind speed measurements appears to be less sensitive to whether the wind is an along or across wind compared to whether the wind is an onshore or offshore wind. This result indicates how important a homogeneous wind field is to the accurate observation of horizontal wind speeds and directions using a scanning LiDAR with the VVP method.  
Figure 11: Scatter plots of (a) wind speeds and (b) wind directions for V1 and 100s for Case 8 (all directions).  
Figure 12: Scatter plots of (a) wind speeds with an along-wind direction (parallel to the LiDAR heading angle \(\pm\)45°) and (b) an across-wind direction (perpendicular to the LiDAR heading angle \(\pm\)45°).