Relationship between Doppler wind lidar observed wind speed and Carrier-to-Noise ratio

We investigate the relationship between the CNR and the mean horizontal wind speed estimated by a Doppler lidar. Intuitively, high values of CNR represent high concentration of particles that backscatter the light beam and therefore denote observations with high accuracy, while decreasing values of the CNR gradually give rise to a higher uncertainty. For a Leosphere V2 Doppler wind lidar that was operated in DBS mode in the high Arctic, we observed three distinct ranges of the normalized mean wind speed along the range of CNR values. In the CNR range from ≈ -14 dB to -22 dB there is a decrease of about 5% in the observed mean horizontal wind speed. It is very fortunate that the decrease is followed by a flat plateau of the wind speed for CNR values between -22 dB and -26 dB in which the estimate of the wind speed is not sensitive to the choice of the CNR threshold value. An abrupt increase in the normalized wind speed for CNR values < -26 dB signifies that the backscattered signal becomes weak due to low aerosol concentrations and noise starts to corrupt the estimation of the mean wind speed. This characteristic pattern was also found in observations at the Meteorological Observatory Lindenberg using a WindCube 200s and a Stream Line XR system, both operating in VAD mode. But the specific CNR values separating the three distinct ranges are not quite the same. Generally, the flat plateau and the subsequent rapid increase as CNR values decrease further are well known and are a consequence of the detector sensitivity with respect to the noise level; this feature can help operators to identify a filter threshold suitable for their particular instrument. However, the decrease of the wind speed for CNR values between -14 dB and -22 dB deserves further investigation.

Based on the two data sets of Doppler wind lidar observations, the decrease in the wind speed for CNR values, as well as the flat plateau will be presented and discussed. The CNR dependence on the observed wind speed is likely instrument dependent but can also have a sensitivity to height as well as meteorological conditions.