# Measurement and Prediction of Wind Fields at an Offshore Site by Scanning Doppler LiDAR and WRF
## 3 Results and Discussion
### Measurement and Prediction of Wind Field in Near-Shore Boundary Layer  
In order to evaluate the effect of coastal terrain on the near-shore marine ABL, the wind field between the meteorological tower and the coastline was measured. RHI scans were conducted towards the north and for an elevation angle range of \(0^{\circ}\) to \(60^{\circ}\). The wind direction information required by Equation (5) to construct wind speeds from LiDAR-measured LOS velocities (\(u_{r}\)) was obtained from a wind vane mounted on the meteorological tower. Furthermore, for the spatial averaging, the measurement data was divided into vertical grids along the north south direction. The size of each block in the grid was set to \(50\times 50\) m\({}^{2}\). Average wind speeds were computed from all the measurement points in that block and for a fairly long time span of three hours.  
Figure 16 shows the mean velocity field in the vertical plane between the observation tower and the northern coast. The left side of the color map is 1000 m from the coast, and the LiDAR is 3500 m from the coast. Strong shear can be observed in the region close to the coast. The coastline has a vertical cliff of about 50 m above the sea level. This enforces the development of an internal boundary layer (IBL) that persists approximately up to 1500 m from the LiDAR position (almost 2 km from the shore).  
Figure 16: Visualization of wind fields measured in the vertical plane from LiDAR position towards the coast. Vertical dashed lines indicate positions where velocity profiles are plotted in Figure 17. Measurement data collected on 28 November 2016 from 12:00 to 15:00.  
Figure 15: Comparison of 10-min-averaged wind speed and wind direction of scanning Doppler LiDAR (retrieved using VAD) and met mast tower at 90 m height (September 2016).  
Figure 17 presents average boundary layer profiles at distances 900 m, 1900 m, and 2900 m from the coastline. All data between 18 November 2016 and 25 December 2016 for which the wind directions were within the limit of \(\pm 10^{\lx@math@degree}\) from the north were averaged to produce these profiles. It can be appreciated that the simulations are able to produce more or less similar velocity magnitudes as the measurements. However, for the profiles at 900 m from the coast, i.e., closest to the coast, the difference between the measurement and simulations is significant. Difference between the simulations and the measurements is around 10% up to 500 m. Note that the height where the error line is broken in Figure 17 corresponds to the location where no measurement data was available. For Figure 17b,c, which are further from the coastline, LiDAR measurements are well reproduced by the profiles predicted by the WRF simulations up to a height of 250 m. However, as was the case for the wind blowing from the land (cf. Figure 9c), for higher altitudes, the simulations overestimate the measurement, for the current measurement period.  
Until now, results from mesoscale simulations were mainly validated against tower-mounted anemometers or profiling LiDAR with a lower range. Therefore, it was only possible to confirm the accuracy of the simulation results at 200 m or below. By using scanning Doppler LiDAR, together with the proposed method for the reconstruction of the velocity field from LOS velocities, it is now possible to validate the mesoscale simulations against measurements for higher altitudes and different locations in the IBL.