Page 39 - Wind Energy Handbook

LONG-TERM WIND SPEED VARIATIONS 13

identifiable features of this such as the trade winds and the ‘roaring forties’ are well
known.
The non-uniformity of the earth’s surface, with its pattern of land masses and
oceans, ensures that this global circulation pattern is disturbed by smaller-scale vari-
ations on continental scales. These variations interact in a highly complex and non-
linear fashion to produce a somewhat chaotic result, which is at the root of the day-
to-day unpredictability of the weather in particular locations. Clearly though, under-
lying tendencies remain which lead to clear climatic differences between regions.
These differences are tempered by more local topographical and thermal effects.
Hills and mountains result in local regions of increased wind speed. This is partly
a result of altitude – the earth’s boundary layer means that wind speed generally
increases with height above ground, and hill tops and mountain peaks may ‘project’
into the higher wind-speed layers. It is also partly a result of the acceleration of the
wind flow over and around hills and mountains, and funnelling through passes or
along valleys aligned with the flow. Equally, topography may produce areas of
reduced wind speed, such as sheltered valleys, areas in the lee of a mountain ridge
or where the flow patterns result in stagnation points.
Thermal effects may also result in considerable local variations. Coastal regions
are often windy because of differential heating between land and sea. While the sea
is warmer than the land, a local circulation develops in which surface air flows from
the land to the sea, with warm air rising over the sea and cool air sinking over the
land. When the land is warmer the pattern reverses. The land will heat up and cool
down more rapidly than the sea surface, and so this pattern of land and sea breezes
tends to reverse over a 24 h cycle. These effects were important in the early
development of wind power in California, where an ocean current brings cold
water to the coast, not far from desert areas which heat up strongly by day. An
intervening mountain range funnels the resulting air flow through its passes,
generating locally very strong and reliable winds (which are well correlated with
peaks in the local electricity demand caused by air-conditioning loads).
Thermal effects may also be caused by differences in altitude. Thus cold air from
high mountains can sink down to the plains below, causing quite strong and highly
stratified ‘downslope’ winds.
The brief general descriptions of wind speed variations in Sections 2.1 to 2.5 are
illustrative, and more detailed information can be found in standard meteorological
texts. Section 9.1.3 describes how the wind regimes at candidate sites can be
assessed, while wind forecasting is covered in Section 2.9.
Section 2.6 presents a more detailed description of the high-frequency wind fluctua-
tions known as turbulence, which are crucial to the design and operation of wind
turbines and have a major influence on wind turbine loads. Extreme winds are also
important for the survival of wind turbines, and these are described in Section 2.8.

2.3 Long-term Wind speed Variations

There is evidence that the wind speed at any particular location may be subject to
very slow long-term variations. Although the availability of accurate historical