# Carrier-to-Noise-Threshold Filtering on Off-Shore Wind Lidar Measurements
## 4 Discussion
Having more data by lowering the CNR threshold will ensure more samples to better estimate the mean wind speed, but the additional measurements will have a higher uncertainty and may contribute to the error in the estimated mean wind speed and shape of the wind speed distribution. This effect was investigated on the off-shore data from FINO3, and the results are presented in Figure 11 as a function of the availability if the data. It can be observed that the mean wind speed is a weak function of the availability of the measurements. This signifies that a high availability is not essential for the estimate of the mean wind speed as long as the measurements are evenly distributed. Figure 11 right panel illustrates that the standard deviation of the wind speed distributions when applying CNR threshold values of -22 dB and -35 dB respectively are only marginally different (-2%) at 126 m and near identical (-0.4%) at 626 m. This is in agreement with Figure 4 where it can be observed that the distributions when applying CNR threshold values of -22 dB and -35 dB are near identical at both at 126 m and 626 m except that the wind speed distribution is shifted towards higher values when the -22 dB CNR threshold is applied as compared to a CNR threshold value of -35 dB. The addition of measurements with a low CNR and, thus, higher uncertainty have only a minor effect on the standard deviation of the wind speed distribution (Figure 7), but the addition of measurements with low CNR have a pronounced effect on the estimated mean wind speed (Figure 5). Figure 10: Mean wind speed at 40 m height (left panel) and availability of the observations at 40 m (right panel) plotted as a function of the filtering height. An availability of 100% corresponds to the total number of measurements at 40 m with a CNR \(\succ\)–22 dB. Because the number of observations with a prescribed CNR threshold or factory setting decreases with height, a CNR filtering is inevitable (see Figure 10 right panel). Therefore, the effect should be investigated when measuring wind climatologies with wind lidars at several heights. It is presently not known how this affects the estimated wind speed and wind direction profiles due to lack of observations that can be used for comparison from meteorological towers (e.g., cup anemometer and wind vane) at the relevant heights. It is found in this study that wind lidar measurements in a marine environment like the North Sea preferably should be carried out with a Carrier-to-Noise-Ratio threshold of -29 dB or lower to obtain a near threshold independent estimate of the mean wind speed and wind rose. Here we deal with the marine atmosphere only, but the sensitivity to the CNR threshold value as found by [31] also to apply to rural, coastal, and urban locations. The generally accepted value of -22 dB is too high at these locations, and it is suggested (see Figure 4 in [31]) also to use the -29 dB or lower as a CNR threshold, or alternatively use all valid observations, until further evidence has been obtained for coastal and on-shore conditions. Although the CNR threshold of -29 dB was found applicable for this lidar, the effect of filtering should always be investigated when deriving wind climatologies from wind lidar observations. Presently, it is hypothesized that the bias is not likely caused by instrument design but, in part, can be ascribed to changes in the aerosol content in the air, which in this study includes particles from sea spray as well as clouds in agreement with [PERSON] et al. [28] where strong correlation was found between aerosol backscatter as measured by a ceilometer and the CNR during an experiment in Boulder, U.S. It remains, however, to be investigated if there are any additionally dependence on pulse repetition frequency, pulse averaging, pulse power, pulse width and other parameters as it is the case for the estimate of the uncertainty in the velocity measurements. [PERSON] et al. [25] and [PERSON] et al.