Publishers version - DTU Orbit

Lidar data has been successfully combined with the output from flow-modelling software,
using both linear models (Bingöl et al., 2008, 2009; Bingöl, 2010) and computational fluid
dynamics, CFD (Harris et al., 2010; Pitter et al., 2012). This pragmatic approach generates
measurements equivalent to a “point-in-space” sensor by using the results of flow modelling
to adjust the measured lidar wind speed. This topic will is dealt with elsewhere in this lecture
series, examining possible improvement of lidar resource assessment capability in complex
terrain.
4.6.6 Dependence on backscatter level
Under conditions of high backscatter, the spectrum provides an accurate measure of the
distribution of line-of-sight velocities within the probe volume, weighted according to Eq.
(91). As the backscattering strength drops (usually associated with increased air clarity) this
has a similar effect to raising the detection threshold, and will lead to elimination from the
spectrum of weaker components of velocity. The impact of the system noise floor on the
detailed spectral shape will also be increased. The centroid values 〈VLOS〉 will be unbiased
and independent of threshold level when the spectrum is symmetrical. However, for a skewed
(asymmetric) spectrum the precise value of 〈VLOS〉 will be sensitive to the threshold. Hence
a small difference in measured wind speed is possible between two measurements under
conditions that are identical in every way apart from the level of backscatter. However, there
is no evidence from comparisons so far to suggest that this leads in practice to a significant
discrepancy.
A further possibility to be considered is the effect of saturation (by very strong scattering
returns from thick cloud) of the lidar detector, electronics or signal processing. In the event
that the input signal exceeds these limits, the spectrum will become distorted, possibly featuring
higher harmonic components of the true Doppler frequencies. In practice, the range of
inputs to the ADC can be tailored to accommodate the highest levels of backscatter that will
reasonably be encountered, eliminating the risk of bias.
4.6.7 Beam obscuration and attenuation
Lidar can operate successfully even when part of its scan is obscured. This confers great
flexibility so that the system can easily be located adjacent to masts, buildings or in forests.
Stationary objects pose no major problem other than the loss of wind measurements from the
relevant obscured sector of the scan. Slowly moving objects can also easily be filtered, based
on the magnitude of their Doppler shift.
Intheabovecases,thefittoEq.(100)willnolongercontaindataoverthefull360 ◦ rangeof
φ. Laboratoryexperiments on moving belt targets have indicated that accurate measurements
are obtained even when over half of the scan is obscured. Large errors in the least-squares
fitting process become possible as the obscuration increases yet further; such conditions are
identified and a null result returned.
4.6.8 Wind direction
Forground based, vertically scanningZephIR, the twobest-fit solutionsZephIR obtainsto Eq.
(100) give values of wind direction that are 180 ◦ apart. Selection between the two options is
made with reference to the measurement of wind direction from a ground-based anemometer.
This needs to be in disagreement by over 90 ◦ with the direction at the chosen height for the
incorrect choice to be made. While such a directional shear (veer) is conceivable in highly
complex terrain and at very low wind speed, it is much less likely in the reasonably uniform
conditions of interest for wind energy applications, and the wind direction selection can be
propagated upwards from measurements at several heights. In the event of the wrong choice
beingmade,leadingtoawinddirectionthatisinerrorby180 ◦ ,thevalueofverticalcomponent
ofthewind w willhave the wrongsign.In otherwords,an updraughtwill bewronglyidentified
90 DTU Wind Energy-E-Report-0029(EN)