# Can LiDARs Replace Meteorological Masts in Wind Energy?
## 3 Comparison of LiDAR Measurement with Sonic and Cup Anemometer
### Mean Wind Speed and Turbulence Intensity  
This section presents the 10-min average wind speed, standard deviation and turbulence intensity measured by the sonic anemometers, the cup anemometers and the profiling LiDAR. The study uses sonic anemometers as reference devices because of their high sampling rate and because they are the preferred device in measurements of atmospheric turbulence [20; 23]. As stated earlier, comparisons are conducted using data collected for a period of one year. Furthermore, the minimum threshold of data availability (\(\eta\)) for the LiDAR and sonic anemometers is set to 80% throughout the study. Experience shows that LiDAR data with lower availability contribute to an increase in uncertainty and to a reduction in the overall data quality. Figure 5 presents a comparison of 10-min averaged horizontal velocities (\(\overline{V}\)) measured by the cup anemometer and the V2 LiDAR against measurements collected by the sonic anemometer at 40 m height. Here, \(\overline{V}\) is defined by  
Figure 4: Data availability for the period of one year from April 2018 through March 2019. (**a**) Cup anemometer; (**b**) Sonic anemometer; (**c**) V2 profiling LiDAR. \[\overline{V}=\frac{1}{T}\int_{t}^{t+T}V(t)\,dt=\frac{1}{N_{T}}\sum_{i=N_{t}}^{N_{t +T}}V_{i}, \tag{3}\]  
where \(V(t)\) and \(V_{i}\) are the instantaneous horizontal velocity in the continuous and discrete form, \(T\) is the average time period (10 min in this study), and \(N_{T}\) is the total number of data in the 10-min time period. While the cup anemometer directly measures \(V_{i}\), for the sonic anemometer and the LiDAR, \(V_{i}\) is computed using \(V_{i}=\sqrt{(u_{i}^{2}+v_{i}^{2})}\), where \(u_{i}\) and \(v_{i}\) are wind speed components in the west to east and south to north directions, respectively. Comparison between the cup and sonic anemometers in Figure 5a shows good agreement. The regression line obtained from linear least squares fitting indicates the uncertainties in the comparison. The slope of the line is 1.0, while the offset is 0.022 m/s. Coefficient of determination \(R^{2}\) of 0.998 can be considered to be significantly high. There are a few outliers for lower wind speeds, i.e., when \(\overline{V}\) from the sonic anemometer is less than 5 m/s. This may be due to the fact that uncertainties in the cup anemometer measurement is higher for lower wind speeds. Nevertheless, even at lower wind speeds, the overall agreement between the cup anemometer and the sonic anemometer is fairly good. Note that the number of data pairs used for this comparison is 40375. Comparison between the V2 LiDAR and the sonic anemometer in Figure 5b shows a larger variation, although the slope of the regression line is still 1.0 and \(R^{2}\) is 0.993 which is still quite high. Conspicuous differences between the LiDAR and the sonic anemometer measurements can be observed at higher wind speeds, i.e., \(\overline{V}\) of the sonic anemometer is larger than 15 m/s. The LiDAR-measured wind speeds are lower at higher wind speeds for this height. One possible reason that was initially considered was that the site experiences high wind speeds predominantly in winter when snowfall is frequent. Because the horizontal speed of snow would usually be lower than the actual wind speed, the laser beam reflected from the snow particles would show a lower wind speed. However, this underestimation in LiDAR measurement is not observed from the comparison of wind speed at 57 m height (cf. Figure 9a). Another source of this discrepancy could be due to interference of the tower wake. However, this would have reduced the wind speed of the sonic anemometer and not the LiDAR. However, the agreement between the LiDAR and the sonic anemometer, with respect to the mean wind speed, is acceptable. The number of data pairs in this comparison is 30,959, which is approximately 10,000 less than for the sonic anemometer and cup anemometer comparison. This is due to the lower data availability of the LiDAR measurement.