Winds in Complex Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 89

Fig. 4.11 Principal sketch of
wind conditions over a ridge.
Modern wind turbines are
usually in the outer layer of

the ridge-crossing flow

4.4 Spectra

The frequency dependence of the power of turbulent fluctuations is described by
turbulence spectra as introduced in Sect. 3.3 . Special turbulence spectra over
complex terrain are given in Panofsky et al. ( 1982 ) and Founda et al ( 1997 ).
Founda et al ( 1997 ) found good agreement between measurements over a hill top
and the spectra given in Eqs. ( 3.78 )-( 3.80 ), because it turned out to be difficult to
determine an appropriate value for the friction velocity. Founda et al ( 1997 ) used
L i x = 1/(2P)/k i p

instead of ( 3.84 ).

4.5 Diurnal Variation

The diurnal variation of the wind speed over ridges and mountain crests resembles
the diurnal variation in the Ekman layer, because these crests are usually above the
surface layer at night-time. Hills and lower mountains which are smaller than the
boundary layer height may develop a shallow surface layer over them as long as
they are quite smooth. Due to the change between boundary layer conditions at
daytime and free-atmosphere conditions at night-time, wind speeds over crests are
usually higher at night-time than at daytime.

4.6 Summary for Complex Terrain

The main peculiarity of flow over a hill or a mountain chain is the speed-up of the
wind speed over the summit or the crest line. The boundary layer over the crest can
be separated into two layers. There is a rather thin inner layer within which
frictional forces dominate over inertial forces. This layer has a depth of typically