# Can LiDARs Replace Meteorological Masts in Wind Energy?
## 3 Comparison of LiDAR Measurement with Sonic and Cup Anemometer  
In this section, the wind data measured by the LiDAR is compared against the measurements from the sonic and cup anemometers for the period of one year from April 2018 through March 2019. First of all, comparisons of the mean wind speed and the turbulence intensity measured by the three devices for the entire year is discussed. Considering the fact that the peak wind speed is required for the estimation of the extreme load experienced by wind turbines, the accuracy of LiDAR-measured peak wind speeds is presented next. Finally, the section discusses the effect of terrain on LiDAR-measured wind speeds. Before discussing the comparison in detail, Figure 4 shows the data availability for the cup anemometer, the sonic anemometer and the LiDAR at 40 m height. Here, the availability (\(\eta\)) is the ratio of the actual number of data measured (\(N_{\text{meas}}\)) by the instrument to the maximum number of data (\(N_{\text{max}}\)) that the instrument can collect in a 10-min time period, and is defined by  
\[\eta=\frac{N_{\text{meas}}}{N_{\text{max}}}\times 100\%. \tag{1}\]  
For the cup anemometer, the availability was mostly 100%, though there were occasions with 0% availability. Due to the concentration of the large number of data points in Figure 4a, the time slots with \(\eta=0\%\) are not visible. For the sonic anemometer, the data acquisition system at the meteorological mast had trouble and it could not store the measured wind data until the first week of May 2018. Consequently, the availability was 0% for this period. Regarding the LiDAR, it was sent for maintenance from May 18 through July 18 and thus, the availability for this period was 0%. Although \(\eta\) was mostly 100% when the LiDAR was operating, there were occasions when the LiDAR was unable to measure the wind speed (i.e., \(\eta=0\%\)) or the availability \(\eta\) was less than 100%. Note that the maximum number of 10-min data that a device can collect in a one-year time period is 52,560. However, in order to collect the maximum number of data, a device has to operate ideally for the entire year, which is not possible, and measurement devices usually collect less data. For example, the number of actually measured 10-min average data for the cup anemometer is 46,970. The availability of this 10-min averaged data (\(\eta_{10}\)) can be defined as:  
\[\eta_{10}=\frac{N_{\text{10,meas}}}{N_{\text{10,max}}}\times 100\% \tag{2}\]  
where \(N_{\text{10,meas}}\) is the actual number of 10-min average data and \(N_{\text{10,max}}\) is the maximum possible number of 10-min average data that can be collected in the given period. This makes the yearly availability of the cup anemometer equal to 89%. The missing data of roughly 10% is difficult to observe in Figure 4a, since a large amount of data from the one year period is concentrated in this figure. Similarly, the yearly availability of the sonic anemometer and the V2 LiDAR are 81% and \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sensor** & **Heights [m]** & **Orientations [\({}^{\circ}\)]** & **Sampling Frequency [Hz]** \\ \hline Sonic anemometers & 40, 57 & \(198^{\circ},17^{\circ}\) & 10.0 \\ _SONIC CORPORATION SAT900_ & & & \\ Cup anemometers & \(10,20,30,40,50\) & \(21^{\circ},20^{\circ},19^{\circ},18^{\circ},16^{\circ}\) & 1.0 \\ _Thies Clima (4.3351.10.141)_ & & & \\ Wind vanes & \(10,30,50\) & \(201^{\circ},199^{\circ},196^{\circ}\) & 1.0 \\ _Thies Clima (4.3150.10.141)_ & & & \\ V2 LiDAR & \(40,50,57\), & \(0^{\circ}\) & \(\approx\)1.0 \\ _Windcube WLS7-724_ & 60 to 220 at 20 m interval & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of wind speed and wind direction sensors installed at the FREA test site. For orientation, true north is set to \(0^{\circ}\) and the meteorological convention is followed. 75% respectively. It was possible to compare and analyze data from a shorter measurement campaign when all three devices were operating perfectly and were able to collect measurements with \(\eta=100\%\).