Poster presentation at CanWEA 2011.

1. Bias and Uncertainty of a Lidar Measurement in
Complex Terrain
Matthieu BOQUET1, Céline BEZAULT2
1LEOSPHERE (mboquet@leosphere.fr), 2METEODYN (celine.bezault@meteodyn.com)
Abstract
Remote sensing systems are more and more used during campaign of measurements for wind resource assessments. Pulsed lidars have a proven accuracy on flat terrains
and offshore conditions, while in complex terrain the loss of flow homogeneity can create a sensor bias during the transformation of measured radial wind speed to horizontal
wind speed (in some cases up to 10%). In previous studies, it has been shown that Computation Fluid Dynamics (CFD) enables to compute the topographical effects on the
wind flow over complex terrain and bring a corrective parameter to the lidar data.
The post-correction methodology raises however some questions as the influence of the model calibration on the correction performances. We propose to study the
sensitivity of the correction factor to several model parameters, by varying the topographical data like roughness, forest density and atmospheric stability.
Indeed, it is studied the correction of bias in lidar data introduced by terrain complexity and the uncertainty added through the application of post-correction
methodologies.
Methodology
• Acciona, as a collaborative partner and owner of the CFD software and of the lidar, has provided data from a met mast and the lidar measured on a complex terrain site.
• Meteodyn has run CFD computations over the site under various calibrations and provided the corrected lidar data to LEOSPHERE.
• LEOSPHERE has analyzed the results. The corrected wind speeds are compared to the anemometer’s measurements at the three heights and mean deviations resulting
linear orthogonal regressions performed at every height are compared to each other to study their variation against the model calibration.
Description of the Case study
A 66m met mast equipped with calibrated cup and sonic anemometers according to IEC standards is located on a complex terrain near the sea and a pulsed lidar is installed
at about 5 meters from the mast. Elevation difference between the two instruments is less than 1m. 4 months of data from October 2010 to February 2011 are used in the
study. The main wind direction South-East is studied.
Correlation of 10min
horizontal wind speed
(blue scatter plot) and
calculated lidar to cup
mean deviation in
percentage (red scatter
plot)
South-East wind direction - height 70m
Correlation of bin-
averaged horizontal
Orographical data – 3D view Horizontal and vertical mesh for 90° wind direction wind speed (blue
scatter plot) and
calculated lidar to cup
Anemometer heights 33.5m 63.9m 66m
mean deviation in
Lidar heights 40m 60m 70m percentage (red scatter
Bias South-East ~9.5% -8.5% -4.6% plot).
Results
By varying the site parameterizations 11 scenarii have been created in order to test the sensitivity of the Lidar module: roughness and canopy height vary from 1.5 to 3
meters, forest density from high to low density, and atmospheric stability from stable to neutral stability. Results are interpreted according to:
• Speed-up factor above the lidar (close to cup position)
• Inflow angle above the lidar
• Difference of inflow angles at the lidar measurement points (4 points distant from a few tens of meters): this value is used for the correction. A low variation of it means a low
variation of the corrected data
• Mean deviation between corrected data and cup
Scenarii 1 2 3 4 5 6 7 8 9 10 11
Roughness 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.4 0.4 0.4
Forest density H L N N N H L L H L N
Atmospheric stability 2 2 2 0 2 2 0 2 2 2 2
Corrected lidar data for
Point at the vertical of the Lidar Average Std South-East wind direction - height 70m scenario 5
70m Speed-up factor (-) 0.97 1.36 1.36 0.97 1.16 1.14 0.99 1.19 1.06 1.16 1.12 1.13 0.137
70m inflow (°) 0.10 0.00 0.20 0.10 0.30 0.40 -0.10 0.10 0.20 0.00 0.30 0.146 0.151
Differences between East & West points of the beams Average std
70m inflow (°) 0.1 0.3 0.2 0.1 0.2 0.1 0.2 0.3 0.1 0.2 0.1 0.17 0.079
Resulting Mean Deviation Average std
70m deviation (%) 0.4 0.8 0.6 0.9 0 -0.4 0.7 0.7 0.3 -0.4 -0.3 0.3 0.496
Bias is here reduced at 70m height from -4.6% to 0.3% in average, with a standard deviation around this mean value of 0.49%, and a resulting deviation ranging from -0.4%
to 0.9%. Accuracy of the corrected data is therefore within the cup and mounting uncertainties expected for a traditional wind measurement campaign on a complex site.
Conclusions References
The information resulting from the CFD calculation and required for the correction is much less sensitive to [1] : Yamada, T, (1983), Simulations of nocturnal drainage flows by a q2l
roughness and atmospheric stability than the speed-up factors and inflow angles. The bias in the measured turbulence closure model, Journal of Atmospheric Sciences, vol. 40,
Issue 1, pp.91-106
data can therefore be reduced keeping the uncertainty of the results as low as required for using the resulting
[2] :A.N. Ross, S.B. Vosper ; Neutral Turbulent flow over forested hills
data in the assessment of the wind resource. Further studies on various site topographies will be conducted [3]: Boquet M. et al.: Innovative Solutions for Pulsed Wind LiDAR Accuracy
to confirm this result. in Complex Terrain, ISARS 2010.
CANWEA 2011, Vancouver