Geoscience Reference
In-Depth Information
Fig. 3.22
Principal sketch of the vertical structure of the ABL in and over forests
between a densely vegetated forest (Fig. 3.1 ) and a sparsely vegetated forest
(Fig. 3.22 ) is that larger air parcels can enter (these movements are sometimes
called sweeps) and leave (also called ejections) the forest canopy sublayer. This
permeability of the rough surface of the forest canopy sublayer leads to an
anomaly featuring higher turbulence intensities in the wake sublayer than expected
from the mean vertical wind gradient in this layer. Therefore, the usual flux-
gradient relationships are not valid in the whole roughness sublayer (see Högström
et al. 1989 for details). This anomalous wake layer may extend to about three to
five tree heights and has many similarities with an urban surface (see Sect. 3.7 for
further details). In contrast to the urban canopy layer, which immerses the entire
vertical extend of the buildings, the forest canopy layer must be subdivided into
two layers in the vertical: the stem layer and the crown layer. In the stem sublayer,
the horizontal wind speed may be higher than in the denser crown sublayer.
Therefore, wind turbines at forest sites should have hub heights of more than
about three times the tree height in order to avoid unnecessary fatigue due to
enhanced turbulence. Together with the large displacement height which comes
with this surface type, this usually means wind turbine hub heights above the
displacement height of considerably more than 100 m, i.e. total hub heights of
about 130-150 m above ground.
3.7 Winds in Cities
Recent increases in urbanisation have resulted in increased urban energy demands.
Research has started to investigate the possibility of local energy generation from
wind by turbines especially suitable for urban environments. Such local energy
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