Page 42 - Wind Energy Handbook

16 THE WIND RESOURCE

The Weibull distribution of hourly mean wind speeds over the year is clearly the
result of a considerable degree of random variation. However, there may also be a
strong underlying seasonal component to these variations, driven by the changes in
insolation during the year as a result of the tilt of the earth’s axis of rotation. Thus in
temperate latitudes the winter months tend to be significantly windier than the
summer months. There may also be a tendency for strong winds or gales to develop
around the time of the spring and autumn equinoxes. Tropical regions also
experience seasonal phenomena such as monsoons and tropical storms which affect
the wind climate. Indeed the extreme winds associated with tropical storms may
significantly influence the design of wind turbines intended to survive in these
locations.
Although a Weibull distribution gives a good representation of the wind regime
at many sites, this is not always the case. For example, some sites showing distinctly
different wind climates in summer and winter can be represented quite well by a
double-peaked ‘bi-Weibull’ distribution, with different scale factors and shape
factors in the two seasons, i.e.,

! !

U k 1 U k 2
F(U) ¼ F 1 exp þ (1 F 1 )exp (2:5)
c 1 c 2

Certain parts of California are good examples of this.

2.5 Synoptic and Diurnal Variations

On shorter time-scales than the seasonal changes described in Section 2.4, wind
speed variations are somewhat more random, and less predictable. Nevertheless
these variations contain definite patterns. The frequency content of these variations
typically peaks at around 4 days or so. These are the ‘synoptic’ variations, which
are associated with large-scale weather patterns such as areas of high and low
pressure and associated weather fronts as they move across the earth’s surface.
Coriolis forces induce a circular motion of the air as it tries to move from high- to
low-pressure regions. These coherent large-scale atmospheric circulation patterns
may typically take a few days to pass over a given point, although they may
occasionally ‘stick’ in one place for longer before finally moving on or dissipating.
Following the frequency spectrum to still higher frequencies, many locations will
show a distinct diurnal peak, at a frequency of 24 h. This is usually driven by local
thermal effects. Intense heating in the daytime may cause large convection cells in
the atmosphere, which die down at night. This process is described in more detail
in Section 2.6 as it also contributes significantly to turbulence, on time-scales
representative of the size of the convection cells. Land and sea breezes, caused by
differential heating and cooling between land and sea, also contribute significantly
to the diurnal peak. The daily direction reversal of these winds would be seen as a
12 h peak in the spectrum of wind speed magnitude.