# Can LiDARs Replace Meteorological Masts in Wind Energy?
## 3 Comparison of LiDAR Measurement with Sonic and Cup Anemometer
### Mean Wind Speed and Turbulence Intensity
Figure 7: Turbulence intensity as a function of mean wind speed at 40 m height. (**a**) Sonic anemometer; (**b**) Cup anemometer; (**c**) V2 profiling LiDAR; (**d**) Bin-averaged turbulence intensities; (**e**) Number of 10-min average data per bin. Lines in (**a–c**) 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. method employed by the profiling LiDAR assumes horizontal homogeneity at the measurement height. However, the distance between the four measurement points increases with increasing height, and as a consequence, horizontal homogeneity may not be satisfied. The effect should be more pronounced for the standard deviation measurement. Despite this inherent characteristic of the DBS method, it is interesting to observe that the agreement between the LiDAR and the sonic anemometer measurements is better at 57 m. The number of data pairs used in this comparison is 34,783. Figure 10 presents the distribution of turbulence intensity as a function of mean wind speed at 57 m height. The tendencies are similar to those at 40 m height. The bin-averaged turbulence intensity for the LiDAR is higher than that from the sonic anemometer for the mean wind speed range of 2 to 20 m/s. For reference, turbulence categories for the wind turbine classes defined in the IEC standard [30] are also included in the figure. These standard turbulence intensity values are defined as:  
\[I_{\text{IEC}}=\frac{I_{\text{ref}}(0.75\overline{V}_{\text{h}}+b)}{\overline{ V}_{\text{h}}}, \tag{6}\]  
where \(I_{\text{ref}}\) is the reference value of the turbulence intensity, \(\overline{V}_{\text{h}}\) is the hub height wind speed and \(b=5.6\) m/s. The IEC standard has four categories of reference turbulence intensities--A+ (\(I_{\text{ref}}=0.18\)), A (\(I_{\text{ref}}=0.16\)), B (\(I_{\text{ref}}=0.14\)) and C (\(I_{\text{ref}}=0.12\))--for defining wind turbine classes. Note that for class S, the turbulence intensity and other parameters are specified by the designer [30]. The bin-averaged turbulence intensity values measured by both the sonic anemometer and the LiDAR are less than or similar to the turbulence intensity for category A+. For \(\overline{V}\leq 6\) m/s, the measured values are even lower than category A, although for \(10\leq\overline{V}\leq 15\) m/s, the LiDAR-measured bin-averaged values are slightly larger than category A+. However, the IEC standard specifies that for the normal turbulence model (NTM), the representative value of the standard deviation should be set to the 90 th percentile for the given wind speed [30]. This is shown in Figure 10d. For \(\overline{V}>3\) m/s, the turbulence intensity values--obtained from the 90 th percentile of \(\sigma\)--measured by both the sonic anemometer and the LiDAR are larger than category A+ of the IEC standard. Thus, both the sonic anemometer and the LiDAR measurements show that the site requires 'class S' wind turbines which can withstand the site-specific higher turbulence intensity. It can be appreciated from this figure that the LiDAR-measured turbulence intensities are slightly larger (by about 2% or less) than those measured by the sonic anemometer, although the difference between the two measurement sets is not very significant. Figure 9: Comparison of measurements at 57 m height. (a) 10-min average horizontal wind speed; (b) Standard deviation. The effect of this difference on the wind turbine fatigue load is quantified and discussed in Section 4. Note that all further analyses use measurement data from 57 m height.