Winds in Complex Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 90

1-2 % of the half-width of the hill or mountain chain. Above the inner layer is the
outer layer within which the inertial forces dominate. The fractional speed-up is at
maximum at the boundary between the inner and the outer layer. Modern wind
turbines with hub heights around 100 m and more are usually in the outer layer
(Fig. 4.11 ). Therefore, they are exposed to less vertical wind gradients than over
level terrain. Building even higher towers with larger hub heights thus gives only a
relatively low gain in power yields.
Section 4.2 is valid for gentle hills only. Steeper hills and mountains lead to
non-linear features such as flow separation and other features named in Sect. 4.1
which are not adequately covered by the equations given in Sect. 4.2 . Non-linear
flow features can no longer be derived from analytical relations but require the
operation of numerical flow models. Some non-linearity effects become already
visible in the examples for the flow over an escarpment in Sect. 4.3 . Therefore,
wind assessment in rougher terrain where linearity is no longer assured has to be
done by site-specific numerical model simulations. This Chapter was designed to
point to the main flow features which influence the vertical wind profile over hills
and gentle mountains.

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