Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 69

Figure 11: Wind profi les for neutral, unstable and stable conditions [16].

4.2.3 Infl uence of atmospheric stability
The logarithmic law presented above can be expanded to take atmospheric stability
into account:

k ⎛

⎞

⎛⎞

u * h

uh () = ln − Ψ (9)

⎜ ⎟

⎜ ⎝⎠

z

⎝ ⎠

0

Ψ is a stability-dependent function, which is positive for unstable conditions and

negative for stable conditions. The wind speed gradient is diminished in unstable
conditions (heating of the surface, increased vertical mixing) and increased during
stable conditions (cooling of the surface, suppressed vertical mixing). Figure 11
shows an example of the effect of atmospheric stability when extrapolating a
measured wind speed at 30 m to different heights.

4.2.4 Infl uence of orography
The term orography refers to the description of the height variations of the ter-
rain. While in fl at terrain the roughness is the most dominant parameter, in hilly
or mountainous terrain the shape of the terrain itself has the biggest impact on
the profi le.
Over hill or mountain tops the fl ow will be generally accelerated (Fig. 12). As a
consequence the logarithmic wind profi le will be distorted: both steeper and then
less steep depending on height. The degree of distortion depends on the steepness