Assessment of the Repeatability and Stability of Wind Measurements of More Than 400 Coherent Doppler Lidar Units

Over the last decade, Coherent Doppler Lidars (CDL) have become very popular for a variety of applications like wind energy with high requirements in terms of accuracy and precision. It has been already accepted by IEC and ISO international standards for operational uses thanks to many studies performed worldwide by independent and certification bodies. Usually the CDLs are calibrated and validated by the manufacturer after manufacturing. Optionally such sensors can be validated by a third party. In routine operations each Lidar unit is then validated against a reference Lidar calibrated at the Danish Technical University’s test center in Denmark at least once every two years as recommended by IEC Standards -12-1:2017 Annex L.

The goal of the study is to assess the long term quality of CDLs of one type the Windcube short range wind profiler. The accuracy and the precision of the CDLs is assessed by comparing the 10 minutes averaged horizontal wind speed measurement and horizontal wind direction measurement to the reference CDL. 5 years of calibration data have been used representing 595 datasets and 448 CDL units. All datasets are used (all wind speeds and wind sectors).

Over the entire dataset of accuracy and precision results, the overall values are computed for every parameter calibrated and for every height. For example, the mean accuracy of horizontal wind speed at 40 meters is -0.01 m/s but this varies by ±0.03 m/s for all systems.

All available validation datasets are used to evaluate the repeatability of the Windcube, by assessing the maximum variability of accuracy and precision over all available datasets across all heights. A maximum variability of horizontal wind speed accuracy of 0.037 m/s is observed. As well, precision varies at most by 0.034 m/s. Maximum variability of wind speed overtakes the requirements of the industry validating the CDLs as very accuracte and precise wind sensors. A maximum variability of horizontal wind direction accuracy of 1.5° is observed. As well, precision varies at most by 1.6°. Maximum variability of wind direction is also in line with industry best practices.

All client-owned CDL units returned to Leosphere which only went through a quick service check (no changes impacting metrology in between) are compared from the year 0 to each years when they went through the quick check, conceptually mimicking a pre- and post-calibration. This part of this study is for assessing the stability of the wind measurements with time. The average accuracy of Windcube lies between -0.01m/s and 0.01m/s. The average precision of Windcube lies between 0.05 m/s and 0.06 m/s. Stability of CDLs measurements has been demonstrated through 3 years of field use.

In addition to this test, frequent campaigns have been done at worldwide reference test site like Lindenberg observatory or Danish Technological University in Europe to be validated against reference sensors like met masts, radiosoundings or radar wind profilers. The study will present over the last ten years, the results obtained for three campaigns of 2 months each.

Finally, the study allow to demonstrate the long term accuracy and precision that can be reached by CDLs for meteorology related applications and their potentiel use for climate and weather observing networks.