CHARACTERIZING THE OBSERVED WIND RESOURCE - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 132

140

120

100

80

60

40

20

0.35 0.20 0.14 0.08

0

0.0 2.0 4.0 6.0 8.0 10.0

Wind speed (m/s)

Figure 10-2. Theoretical profiles of wind speed with height for a range of values of the exponent
α (0.08, 0.14, 0.20, and 0.35). All curves assume a speed of 8.5 m/s at 120 m height. Source :

AWS Truepower.

Table 10-3. Typical shear exponents for different site conditions. These estimates
may not be valid for specific sites; measurement is required

Approximate range of annual

Terrain type Land cover mean wind shear exponent

Flat or rolling Low to moderate vegetation 0.12-0.25

Flat or rolling Patchy woods or forest 0.25-0.40

Complex, valley
(sheltered)

Varied 0.25-0.60

Complex, valley (gap or
thermal flow)

Varied 0.10-0.20

Complex, ridgeline Low to moderate vegetation 0.15-0.25

Complex, ridgeline Forest 0.20-0.35

Offshore, temperate Water 0.10-0.15

Offshore, tropical Water 0.07-0.10

Source: AWS Truepower.

decreases. This is evident from the following equation for the error in the shear
exponent:

log + ε)

log h 2
h 1

( 1

α =

(10.7)

where is the error in the ratio of speeds measured by the top and bottom

anemometers. For example, for anemometers at 40 and 60 m height (a height ratio

ε