## 5 Data Collection and Operational Performance Validation at Sea  
The lidars were mounted on nine offshore platforms and the island of Utsira. In Table 4 the height of the instrument mounting on the platforms and observational heights are listed in meters AMSL. The observational heights at or near 100 m are shown in italic type in Table 4. These heights are, for simplicity, described as at around 100 m hereafter. The ten lidars were deployed for a total of 91.603 h. For the 77.491 h of data the average data availability was 89%.  
The system and data availability at around 100 m for the lidars is provided in Table 5. The system availability is defined as the time from the mounting to dismounting of the lidar. The downtime in system availability was due to different reasons. One was a mechanical failure of the rotation of lens/mirror in one instrument. In other cases the wiper motors and assembly had problems. They were  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Lidar**} & \multirow{2}{*}{**Time**} & \multicolumn{5}{c}{**Slope at**} & \multicolumn{5}{c}{**R\({}^{2}\)at**} & \multirow{2}{*}{**N**} \\ \cline{3-3} \cline{5-10}  & & **116 m** & **100 m** & **80 m** & **60 m** & **116 m** & **100 m** & **80 m** & **60 m** & \\ \hline \multirow{4}{*}{2} & Pre & 0.988 & 0.992 & 0.991 & 0.993 & 0.999 & 0.999 & 0.998 & 0.998 & 3.606 \\  & Post & 0.991 & 0.997 & 0.993 & 0.992 & 0.999 & 0.998 & 0.998 & 0.997 & 3.659 \\  & Diff. & 0.003 & 0.005 & 0.002 & \(-\)0.001 & 0.000 & \(-\)0.001 & 0.000 & \(-\)0.001 & - \\  & Pre & 0.996 & 0.999 & 0.998 & 0.998 & 0.999 & 0.999 & 0.999 & 0.998 & 5.065 \\
4 & Post & 0.989 & 0.994 & 0.989 & 0.999 & 0.998 & 0.998 & 0.998 & 0.997 & 3.510 \\  & Diff. & \(-\)0.007 & \(-\)0.005 & \(-\)0.009 & 0.001 & \(-\)0.001 & \(-\)0.001 & \(-\)0.001 & \(-\)0.001 & - \\  & Pre & 0.985 & 0.993 & 0.992 & 0.993 & 0.999 & 0.998 & 0.998 & 0.997 & 991 \\
5 & Post & 0.983 & 0.987 & 0.984 & 0.992 & 0.999 & 0.999 & 0.998 & 0.998 & 2.791 \\  & Diff. & \(-\)0.002 & \(-\)0.005 & \(-\)0.008 & \(-\)0.001 & 0.000 & 0.000 & 0.001 & 0.001 & - \\  & Pre & 0.976 & 0.980 & 0.977 & 0.989 & 0.995 re-fitted when possible. The data availability is defined as the instrument is operational but may not (fully) observe data. One reason was that the wiper spraying was working fine but for some instruments the back and forth movement of the wiper caused sometimes a pull backward of water on the lens. For WindCubes there were less data to build up to the 10-min value during wipe. Very low aerosol concentrations may have resulted in lower data availability at the upper levels compared to lower levels but it was not often the case in the North Sea.  
The carrier-to noise ratio (CNR) and availability for some WindCube lidars is shown in Figure 6. The observations show the lower signal with higher observational height due to weaker signal and mainly because of lower aerosol content at the higher heights. The maximum CNR is around 100 m or slightly below. The CNR behavior with height shows a peak because of the focus of the system [20].  
The observations at Siri were similar in quality to the other instruments from June 2010 to 18 November 2010. After this time the lidar suffered from a still unknown issue preventing to record good data above its 40 m level during night time.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Platform** & **Babbage** & **Beatrice** & **Fino3** & **HornsRev2** & **Jacky** & **ORP** & **Schooner** & **Siri** & **Taqa** & **Utsira** \\ \hline Lidar type & ZP & ZP & ZP & WC & WC & WC & WC & WC & WC & WC \\ Height & 42 & 42.5 & 26 & 26 & 28 & 30 & 36 & 45 & 30 & 26 \\
1 & 60 & 52.5 & 51 & 66 & 60 & 70 & 76 & 85 & 70 & 67 \\
2 & 80 &