ESTIMATING THE RESOURCE AT HUB HEIGHT - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 144

0.40

0.35

0.30

0.25

0.35

0.20

0.15

0.20

0.10

0.14

0.08

0.05

0.00

0 5 10 15 20

Displacement height (m)

25 30 35

Figure 11-1. Effect of displacement height on the shear exponent between the top height of a
mast and the hub height, for different values of observed shear. The observed shear is calculated
between 40 and 60 m, and the hub height is assumed to be 80 m for each case. Source :AWS
Truepower.

should be reduced as the distance between the vegetation and the tower increases.
Unfortunately, there are no proven guidelines for estimating this reduction. In the
absence of such guidelines, a linear reduction over a distance of 20-50 times the
vegetation height is reasonable.
As a practical matter, if the vegetation is no more than a few meters tall, the
displacement has little impact and can safely be ignored. It is only in forest with trees
more than about 10 m tall that the effect of displacement on the expected shear above
the top height of the mast becomes significant.
For towers with at least four measurement heights, it is sometimes possible to
dispense with displacement height and fit the observed change in shear directly to the
data. A logarithmic function often works well for this purpose. This approach is rarely
feasible, however, because few towers have as many as four levels. In addition, if the
lowest level is sheltered by trees or obstacles, the fitted curve may not be reliable.

11.1.3 Convergence Height
A characteristic of the atmospheric boundary layer is that the influence of variations in
topography and land cover tends to diminish with height. This idea is neatly captured
in the concept of a convergence height, which is defined as the height above ground
where the wind speed profiles at different points in a project area converge and the
wind resource becomes homogeneous.
There is no standard or easily calculated convergence height: it varies greatly
depending on the site and atmospheric conditions. In nearly flat, featureless terrain,