# Data Processing and Analysis of Eight-Beam Wind Profile Coherent Wind Measurement Lidar
## 3 Results and Discussion
### Contrast Observations  
Generally, a set of radial velocity measurements are combined with appropriate assumptions to extract useful information for wind field analysis, such as the wind speeds and directions [21]. A 30-min average profile of the wind vector was calculated based on the assumption of a horizontally homogeneous wind field. There is a degree of correlation between the symmetrical beam measurements [41; 54]. The combined calculation of the wind field using the eight-beam points can overcome the pollution of the individual beam effectively and can improve the quality of the calculated data. In order to verify the performance of the eight-beam wind profile coherent wind measurement Lidar system, a comparative experiment of wind measurement using wind profile Lidar was carried out successfully. Over 48 h of data were obtained for the entire comparison experiment. Figure 9 shows a comparison of the horizontal winds at 7:30 a.m. on 5 March 2021. The quality-controlled Lidar measurements were compared with independent reference data from two collocated operational Lidar wind profiles running in three-beam and five-beam Doppler beam swinging modes. The parameters of the instruments used for the three experiments were the same. The directions of the three-beam wind measurements were north (0\({}^{\circ}\)), east (90\({}^{\circ}\)), and vertical. The directions of the five-beam wind measurements were north (0\({}^{\circ}\)), east (90\({}^{\circ}\)), south (180\({}^{\circ}\)), west (270\({}^{\circ}\)), and vertical. During the experiment, an elevation angle of 60\({}^{\circ}\) and a range resolution of 30 m were used for the measurements. A combined vertical profile of the magnitude and direction of the horizontal wind speed was obtained every half hour. The main purpose of the experiment was to compare the magnitudes and directions of the horizontal wind fields measured using the three techniques in the same time and space domains under the same weather conditions. Figure 8: Profiles of the horizontal wind speed and direction from 00:00 to 01:30 on 6 March 2021. As shown in Figure 9, the wind profiles obtained using the different techniques exhibit good consistency. Within the height range of 1 km, from 7:00 to 7:30 in the morning, the maximum values of the standard deviation of the magnitude and direction of the horizontal wind speed measured using the three instruments were 2.44 m/s and 43.15\({}^{\circ}\), respectively, with average values of 0.42 m/s and 6.70\({}^{\circ}\), respectively. These errors include the systematic measurement errors and the errors caused by atmospheric changes during the 30-min measurement period. At altitudes greater than 700 m, the difference in the wind speeds measured using the eight-beam and other techniques was observed, especially at altitudes of 800-1000 m.  
The experimental results show that the standard error was less than 0.42 m/s compared with the results obtained from the three-beam and five-beam wind measurements. As can be seen from Figure 10, the absolute error varied with height. The average absolute error between the eight-beam and three-beam techniques was 0.34 m/s at heights of <300 m. The average absolute error between the eight-beam and five-beam techniques was \(-\)0.094 m/s at heights of <300 m. A positive number indicates a high wind speed measured by the eight-beam, and vice versa. At this time, the accuracy of the wind measurements was greatly affected by the cluttering near the ground objects. The average absolute error between the eight-beam and the three-beam techniques was 0.01 m/s below heights of 300-700 m. The average absolute error between the eight-beam and the five-beam techniques was 0.23 m/s below heights of 300-700 m. During this period, the influence of the near-ground cluttering decreased significantly, and the results obtained using the eight-beam technique are in good agreement with those obtained using the other techniques.