# Carrier-to-Noise-Threshold Filtering on Off-Shore Wind Lidar Measurements
###### Abstract  
Wind lidar observations are characterized by a Carrier-to-Noise-Ratio that is often used to filter the observations. The choice of the Carrier-to-Noise-Ratio threshold value for the wind lidar observations is found to have an effect on the climatological wind speed distribution in such a way that when the Carrier-to-Noise-Ratio (CNR) threshold value is increased the wind speed distribution is shifted to higher values. Based on one year of observations carried out with a wind lidar from 126 m to 626 m height at the FINNO3 (Forschungspalttform in Nord- und Ostsee Nr. 3) research platform in the North Sea, the effect that the choice of the Carrier-to-Noise threshold value has on the climatology of the wind speed and direction as well as the wind power density in relation to wind energy is illustrated and discussed. In the one-year data set considered here it is found that for thresholds larger than \(-29\) dB, the mean wind speed and wind rose measured by the wind lidar become a function of the threshold value, and for values smaller than \(\sim-29\) dB further decrease of the Carrier-to-Noise-Ratio threshold has a minor effect on the estimated mean wind speed and wind rose. The analysis of the data set from the North Sea shows that the limit for the Carrier-to-Noise-Ratio of the observations should be \(-29\) dB or less to obtain a threshold independent estimate of the mean wind speed and wind rose. Alternatively, all valid observations should be used for the analysis. Although this study is specific for the conditions in the North Sea, we suggest that for a representative estimation of the wind resource with wind lidars, the effect of the CNR threshold filtering on the wind distribution should be studied when the recovery rate is less than 100%. Carrier-to-Noise-Ratio (CNR); heterodyne doppler wind lidar; off-shore; climatology wind speed; direction and power density +
Footnote †: journal: Sensors  
0000-0002-4000-0000-0000-0000-00000-0000-00000-0000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-000000-0000  
preference since its appearance as a commercially available unit, although still not fully recognized as a bankable alternative to the traditional wind speed measurements by cup-anemometers. In this study, off-shore observations of the wind profile up to 626 m performed with a Doppler wind lidar are analyzed. A Doppler wind lidar can, under favorable conditions, reach higher up. [PERSON] argued in [20] that if the so-called Carrier-to-Noise-Ratio (CNR) falls below a prescribed threshold value, the uncertainty in the wind speed is too large for the measurements to be useful; he recommended CNR \(>\)\(-\)22 dB. The uncertainty of the individual observations by the wind lidar depends on the CNR in such a way that high values of the CNR threshold value are associated with observations of predominantly high wind speeds with a low uncertainty and decreasing threshold values are associated with increasing uncertainty of the observed wind speed [21; 22; 23; 24]. This observation will be further developed in this study. A method to compensate for the instrument noise which reduces the signal-to-noise ratio threshold value that is used as a measure of the data quality is presented in [25]. Along this line [26] suggests a post-processing algorithm that provides a better estimate of the signal-to-noise-ratio of the observations, which results in a more accurate estimate of the uncertainty of the line-of-sight measurements. Presently the algorithm is implemented for a Halo Doppler Wind Lidar in Finland and is shown to improve the sensitivity of the observations with at least a factor of five. Alternatives to the CNR threshold data filtering approach have been suggested by [PERSON] [9] and [PERSON] and [PERSON] [27], based on dynamic data-filtering.