LIDAR systems are getting more and more accurate and
reliable. It has been shown many times that the mean
horizontal wind speed measured by a lidar over flat terrain
compares very well with that measured by a cup anemometer.
But can a lidar measure turbulence?
Here we investigate the case of a continuous wave,
conically scanning Zephir lidar. First, the wind speed
standard deviation measured by such a lidar gives on average
80% of the standard deviation measured by a cup
anemometer. This difference is due to the spatial averaging
inherently made by a cw conically scanning lidar. The spatial
averaging is done in two steps: 1) the weighted averaging of
the wind speed in the probe volume of the laser beam; 2) the
averaging of the wind speeds occurring on the circular path
described by the conically scanning lidar.
Therefore the standard deviation measured by a lidar
resolves only the turbulence structures larger than a length
scale depending on the circle diameter and the mean wind
speed (range of magnitude: 100m). However, the Zephir lidar
gives another turbulence quantity, the so-called turbulence
parameter, which can resolve turbulence structures with a
smaller length scale.
In this paper, we suggest a volumetric filtering of the
turbulence to represent the effect of the spatial averaging
operated by a lidar when measuring the wind speed. We then
evaluate this model by comparing the theoretical results to
experimental data obtained with several Zephir systems, for
both turbulence quantities.
|Series||Denmark. Forskningscenter Risoe. Risoe-R|
- Wind energy
- Test and measurements