# Measurement and Prediction of Wind Fields at an Offshore Site by Scanning Doppler LiDAR and WRF
## 3 Results and Discussion
### Measurement and Prediction of Vertical Wind Profiles

Table 2: Comparison of the measurements of scanning LiDAR and V1 LiDAR at 140 m height for CNR threshold of \(-25\) dB and different availability limits. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\ \begin{tabular}{c} **\(\eta_{\text{lim}}\) (\%)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Minimum** \\ **Number of Data** \\ \end{tabular}  } & \multirow{2}{*}{**Offset**} & \multirow{2}{*}{**Slope**} & \multicolumn{2}{c}{**Coefficient of**} & \multirow{2}{*}{**RMSE** **(m/s)**} \\  & & & & & **Determination (\(R^{2}\))** & **(m/s)** \\ \hline
0 & 1 & 0.14 & 0.996 & 0.9549 & 0.924 \\
20 & 28 & 0.13 & 0.996 & 0.9607 & 0.868 \\
40 & 56 & 0.126 & 0.99 & 0.9612 & 0.844 \\
60 & 82 & 0.124 & 0.996 & 0.9672 & 0.806 \\
80 &Here, \(N_{10,\eta_{\rm lim}}\) is the number of 10-min-averaged data for \(\eta\geq\eta_{\rm lim}\), and \(N_{10,\ \rm max}\) is the maximum possible number of 10-min-averaged data for the given time period. As shown in the figure, if \(\eta_{\rm lim}\) is set high, \(\eta_{10}\) becomes smaller, and vice versa. For example, when \(\eta_{\rm lim}\) is set to 80%, \(\eta_{10}\) is around 40% at a height of 500 m, whereas, when \(\eta_{\rm lim}\) is set to 20%, \(\eta_{10}\) is around 70% at the same height. Therefore, although setting a higher \(\eta_{\rm lim}\) improves the quality of the average wind speed data (see Table 2), the number of data decreases significantly. This effect is more pronounced for higher altitudes. Figure 9 presents one-month-averaged boundary layer profiles for the wind blowing from the land and that from the sea obtained from the LiDAR measurements, as well as from the mesoscale simulations. The definition of the wind blowing from the land and from the sea is based on the coastline feature of the test site (cf. Figure 1). The simulation results show a good agreement with the measurements for the wind blowing from the sea, with the maximum error of 2.3%. Regarding the wind blowing from the land, the velocity profile obtained from the simulation reproduces the measured profile up to a height of 300 m. However, for higher altitudes, the simulation overestimates the measurement by up to 10%. The largest difference between the simulation results (due to grid resolution) was observed at 490 m, and it was about 0.05%. However, the differences between simulations with the three grid resolutions are negligible. In Figure 9, the wind speed is higher when the wind is blowing from the sea than when it is blowing from the land. For example, around the hub height level (\(\approx\) 80 m), the wind speed when the wind is blowing from the land is 6.8 m/s, whereas that of the wind blowing from the sea is 9.2 m/s, with the latter being 35% higher. This trend is also observed in the diurnal cycle of vertical profiles in Figure 10. The figure shows hourly averaged vertical profiles, which are further averaged over the period of one month. With very few exceptions, profiles for the wind blowing from sea have higher wind speeds. Thus, the effect of wind direction on power production can be significant for near-shore wind farms and must be taken into account while deciding the layout of such farms. Note that this work does not consider boundary layer stratification. Dividing the measured data into stable, neutral, and convective boundary layers and then analyzing the wind blowing from sea and land may present a further detailed picture of the ABL at the site, as shown by [PERSON] et al. (2018). Figure 8: (**a**) Number of data measured every 10 min at different heights for the period of two weeks from 1 April 2016 to 15 April 2016. Horizontal lines indicates \(\eta_{\rm lim}=80\%\). (**b**) Data availability profiles for various \(\eta_{\rm lim}\) values. Horizontal line indicates the hub height of the wind turbine installed at the site. Figure 10: Comparison of the diurnal cycle of vertical profiles of horizontal velocity for wind blowing from the land and that from the sea (for May 2016).