# An Inter-Comparison Study of Multi- and DBS Lidar Measurements in Complex Terrain
## Materials and Methods
### Data Treatment, Quality Control and Coordinate Systems  
Quality control of lidar data is usually done by using a lower carrier-to-noise ratio (CNR) threshold to identify periods when not enough backscatter is received to derive a reliable wind-speed estimate. If a structure is present close to the measurement volume, returns from this hard target can also contaminate the signal. Often this leads to an increased CNR and/or to an increased scatter in the observed CNR values. Careful analysis of the WindScanner data, which was collected during the presented experiment, however, showed that also periods during which the CNR fell into the expected CNR range (here \(-27.5\) and \(-5\) dB were used) and had a \"normal\" variability sometimes exhibited spurious radial wind-speed statistics. Therefore, two additional filter criteria were imposed on the WindScanner data. The first sorts the radial wind velocities of each averaging interval according to their magnitude. Outliers are then removed if the gap between them and the rest of the data exceeds 1 m\(\cdot\)s\({}^{-1}\). This method is useful in situations with a relatively high radial velocity which is contaminated with hard target returns, as these are often characterised by a radial wind speed close to zero. The filter, however, becomes ineffective if the mean radial wind speed is close to zero--i.e., winds perpendicular to the lidar beam or low wind speeds. The second filter criterion therefore checks the time series during the averaging interval for its temporal consistency. If the time series after the application of the first filter still exhibits a difference between two adjacent measurements which exceeds 1 m\(\cdot\)s\({}^{-1}\), the respective period is removed completely from further analysis. This threshold seems justified because of the relatively high sampling frequency (0.5 Hz) and the damping effect of the lidar probe volume on small-scale turbulence. The exact reason for the problems with the hard targets could not fully be clarified. At the beginning of the experiment, the WindScanners were focused within \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **MA** & **EE** & **SE** & **SW** & **WC** \\ \hline
**Instrument type** & **WLS 2005 V2** & **WLS 2005** & **WLS 2005** & **WLS 2005 V2** & **WINDCUBE WLS7 V2** \\ \hline Altitude (m) & 387.7 & 294.6 & 346.6 & 258.3 & 387.7 \\ Latitude (m) & 5,690,182.6 & 5,690,409.1 & 5,688,371.9 & 5,687,503.8 & 5,690,181.2 \\ Longitude (m) & 513,590.5 & 514,213.3 & 516,843.2 & 512,185.9 & 513,593.1 \\ \(\phi\) (\({}^{\circ}\)) & 90 & 23.3 & 3.5 & 6.0 & 62, 90.9 \\ \(\mathrm{t}\) (\({}^{\circ}\)) & - & 250.6 & 299.3 & 27.4 & 8,98, 188, 278. - \\ Dist. to mast (m) & 2 & 732 & 3740 & 3047 & 3 \\ Pulse length FWHM (ms) & 100 & 400 & 400 & 400 & 175 \\ Gate length FWHM (ms) & 74 & 150 & 150 & 150 & 58 \\ \(\varphi_{\mathrm{r}}\) (m) & 6.4 & 25.5 & 25.5 & 25.5 & 11.1 \\ \(\sigma_{\mathrm{z}}\) (m) & 9.5 & 19.1 & 19.1 & 19.1 & 7.4 \\ Accumulation time (s) & 2 & 2 & 2 & 2 & -1.12 (-5.62 for full circle) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the lidar devices and locations which were used in this study. Coordinates are given in UTM Zone 32U. Pulse length and gate length are given as full width at half maximum (FWHM). approx. 1 m above the reference sonic. The following points may then have led to interference with the mast structure: instability of the WindScanner systems (sinking into soil after installation, vibration of the systems due to e.g., wind), switching of the staring mode to other patterns inducing pointing error due to backlash of the internal gearing, and/or movement of the mast structure. For future inter-comparison experiments, it is thus recommended to increase the distance to the reference instrument/mast. The problem with the hard targets was not present in the data of the Windcube v2 and only a CNR filter was applied. The values (\(-22\) and \(-5\) dB) slightly differ from the WindScanners as the system configuration is different.