# An Inter-Comparison Study of Multi- and DBS Lidar Measurements in Complex Terrain
## 3 Results and Discussion
### Horizontal Wind-Speed Statistics from the ML and Doppler Beam Swinging (DBS) Technique
#### 3.2.1 First-Order Statistics of the Horizontal Wind Speed
In combination with the large scanning volume of the DBS lidar at 188 m this would introduce an increased complex terrain error. To our knowledge no statistics from a comparably long-term data set for assessing the performance of ML measurements to derive the mean wind speeds have been published so far. Previously published results of mast-lidar inter-comparisons either focused on the evaluation of individual measurement periods [16] or employed scanning strategies that focused more on assessing the spatial heterogeneity of the wind field than on evaluating ML measurements against a reference [17; 40]. The ML-mast inter-comparison in the latter two references is slightly worse, but due to the different focus of the scanning strategy, is likely to not be directly comparable. A recently published study by [PERSON] only compares co-located DBS and ML measurements [19]. The sonic anemometer used in this study only measures at significantly lower altitudes than the lidars and is only used to give an indication of the wind field below the lidar measurements. This precludes a quantification of the advantages of either of the techniques over the other in their study or a comparison to the results presented. One of the typical observations for profiling lidars in complex terrain is that the deviation in \(\overline{V_{h}}\) between the lidar and the mast measurements varies with the shape of the terrain and thus wind direction [4; 6]. This behaviour can also be observed for the DBS data collected during the _Kassel Experiment 2014_ (Figure 6). The pattern clearly reflects the shape of the terrain and is very similar to what has been observed in earlier inter-comparisons between cup anemometers and a DBS lidar at Rodeser Berg [6]. If the flow is directed across the ridge of the hill (approx. 50\({}^{\circ}\)-100\({}^{\circ}\) and 200\({}^{\circ}\)-270\({}^{\circ}\)), the DBS lidar underestimates \(\overline{V_{h}}\). This is a typical observation for convex flow as observed over e.g., ridges or hilltops [4; 5]. In contrast, the sectors showing an overestimation of \(\overline{V_{h}}\) by the DBS lidar (approx. 290\({}^{\circ}\)-20\({}^{\circ}\) and 160\({}^{\circ}\)-180\({}^{\circ}\)) correspond to along the ridge flow. Here, the mast is located on the slope in front of/behind the highest point on the ridge and the terrain is slightly concave. This might also cause a concave flow pattern which results in an overestimation of the horizontal wind speed by the DBS lidar. Some evidence for this can be found in the change of sign of the flow angle measured by the sonic between 290\({}^{\circ}\)-20\({}^{\circ}\) and 160\({}^{\circ}\)-180\({}^{\circ}\). Whereas for the first sector (mast in front of the hill top) the median flow angle is 1.7\({}^{\circ}\) (directed upwards), it is \(-2.2^{\circ}\) (directed downwards) for the second sector (mast behind the hill top). It should be noted, however, that the real flow might be more complex than this simplified explanation. A more detailed discussion and modelling results of directional errors of DBS lidar measurements at Rodeser Berg can be found in [6]. Figure 5: Scatter plots of \(\overline{V_{h}}\) from different lidar configurations against the reference sonic at 188 m; (**a**) SE/SW/EE; (**b**) SE/SW; (**c**) SE/EE; (**d**) SW/EE; (**e**) Windcube v2 next to the mast; (**a–d**) are measurements from WindScanners in ML mode; the Windcube in (**e**) is operated in DBS mode. For ML combinations with only two beams, \(w=0\) m\(\cdot\)s\({}^{-1}\) is assumed in the wind-vector reconstruction; n indicates the number of value pairs displayed in the individual scatter plots. On average, the DBS-mast deviations are slightly shifted (1%-2%) towards an increased \(\overline{V_{hidar}}\) (relative to \(\overline{V_{hsonic}}\)) when compared to the earlier inter-comparison to cup anemometry. This might be caused by the different reference anemometers or the different DBS instruments used in the present study, although the reason could not be fully clarified.