# Can LiDARs Replace Meteorological Masts in Wind Energy?
## 3 Comparison of LiDAR Measurement with Sonic and Cup Anemometer
### Peak Wind Speed  
Peak gust wind speeds are an important parameter for the design of wind turbines or any other structures that experience wind loading. The IEC [30]--the lead organization responsible for the standardization of the wind turbine design process--has several specifications related to gust winds. Although the models for gust defined in the IEC standard or other guidelines for wind turbine certifications (see e.g., the GL Guideline [31]) are simplified models which can be easily implemented in the aeroelastic code for faster simulation, rigorous measurement may be necessary to improve the models or simply to modify them for site-specific conditions. Note that the extreme wind speeds that turbines must be designed to withstand are defined in terms of the recurrence or return period. An extreme wind speed with a recurrence period of \(N_{y}\) years (usually \(N_{y}\) is one year or 50 years) has an annual probability of occurrence of \(1/N_{y}\). Measurement of extreme wind speeds is difficult, since their frequency of occurrence is low. However, it is often a practice to estimate extreme wind speeds  
Figure 10: Turbulence intensity as a function of mean wind speed at 57 m height. (**a**) Sonic anemometer; (**b**) V2 profiling LiDAR; (**c**) bin-averaged turbulence intensities; (**d**) turbulence intensity for the 90 th percentile of the standard deviation for the wind speed bin; (**e**) number of 10-min average data per bin. Lines in (**a,b**) represent the 90 th percentile of the standard deviation for the wind speed bin. Bin size is set to 1 m/s of mean wind speed. from measurements over a shorter period (usually one year) and to use them together with statistical models specified in guidelines to estimate extreme wind speeds with long recurrence periods [29; 30]. Therefore, accurate measurement of peak wind speed is still crucial. However, with the instantaneous wind speed being a random process, the peak or gust wind speed for each 10-min window will also become a random variable. Figure 11 compares the sonic anemometer-measured and LiDAR-measured time series of the peak wind speed (\(\widehat{V}\)) in every 10-min time window for one-year period and at 57 m height. It can be appreciated that \(\widehat{V}\) measured by the sonic anemometer has a larger amplitude compared to that measured by the LiDAR. For example, in the two zoom-in figures, the highest value of the peak wind speed occurs around the same time for both the sonic anemometer and the LiDAR measurements. In the left zoom-in figure, the peak wind speed (pointed by the arrow) measured by the sonic anemometer is 33.3 m/s, while that measured by the LiDAR is 32.3 m/s, i.e., lower by roughly 3% for the LiDAR. Similarly, in the right zoom-in figure, the maximum values of the peak wind speed for the sonic anemometer and the LiDAR are 39.8 m/s and 37.3 m/s respectively, i.e., the LiDAR measurement is 6.3% lower than the sonic anemometer measurement. This is one of the limitations of the LiDAR, and the comparatively lower value of the peak wind speed may be the combined effect of the lower sampling frequency and the characteristics of the DBS mode. However, in wind engineering, it is a common practice to employ a moving average filter with an averaging time of 3 s when evaluating gusts at a site [32]. Figure 12 shows the probability density function (PDF) of \(\widehat{V}\) obtained from the sonic anemometer and the V2 LiDAR. It is interesting to observe that while for \(\widehat{V}<\) 9 m/s, the sonic anemometer has higher PDF values, when \(\widehat{V}\) is larger than 9 m/s, either the V2 LiDAR has larger PDF value or the values are almost the same for both devices. This indicates that the probability of occurrence of higher \(\widehat{V}\) is similar for both the sonic anemometer and the LiDAR. However, this result should be observed with caution, since the number of data sets decreases with increasing \(\widehat{V}\) in the current measurement campaign. Figure 11: Time series of maximum wind speed in every 10-min time slot at 57 m height.