GROUND-BASED REMOTE SENSING SYSTEMS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 111

8.5.5 Flow Inclination and Complex Terrain (Sodar and Lidar)
Since sodars and lidars, unlike some cup anemometers, yield a true horizontal wind
speed, it is necessary to determine if off-horizontal winds are contributing to apparent
biases with respect to anemometer data. In most cases, the difference is less than 1%,
but in extreme slopes, it may be as large as 3%. Since turbines respond only to the
horizontal wind component, any adjustment should be applied to the anemometer data
based on that sensor's response to inclined flow, as described in Chapter 9.
In some cases, particularly over narrow ridgelines and in other complex topography,
flow inclination can be nonhomogeneous over the sodar or lidar measurement vol-
ume. This can cause discrepancies of 3-5% with nearby anemometer readings. The
effect varies with the characteristics of the anemometers and remote sensing units,
terrain complexity, land cover, and distance between measurements. This topic is an
area of continuing investigation. Fully characterizing complex sites and reconciling
anemometer and remotely sensed measurements may require bringing additional tools
to bear, including high-resolution flow modeling and high-frequency 3D point and
volume measurements.
8.5.6 Volume Averaging (Sodar and Lidar)
Both sodars and lidars measure the wind speed in a volume of air, in contrast to the
“point” measurements of anemometers. Each layer measured by sodar (regardless of
the height interval at which speeds are reported) actually represents an integral of
information over a depth of 20 m or more. In layers where there is high wind shear,
the volume averaging can cause the sodar to underestimate the mean speed at the
measurement height by up to 3%.
For lidar, the depth of the volume measured can range from less than a meter
to more than 50 m. The actual depth depends on the lidar type, and may be either
variable or fixed over the entire profile. With greater volume depths, high shear can
introduce a bias similar to that seen in sodar systems.
8.5.7 Distance from Reference Mast
Sometimes, sodars and lidars must be placed a considerable distance from a reference
mast to minimize the mast's interference with the measurements, maximize data recov-
ery, or meet other monitoring needs. In moderate and complex terrain and where there
are significant variations in land cover, this can create apparent discrepancies between
measurements simply because of the distance between the two locations. Assessing the
significance of these discrepancies often requires expert judgment, sometimes aided
by numerical wind flow modeling.
8.6 QUESTIONS FOR DISCUSSION
1. How do lidar and sodar work? Describe the basic operating principles of each.