Geoscience Reference
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subtropics. This circulation is indirect and it is the result of the integral effect over
all the moving cyclones in this belt of temperate latitudes. Effectively, the Ferrel
cells transports warmer air towards the poles near the ground and colder air
towards the tropics aloft. This indirect circulation is maintained by energy con-
versions from potential energy into kinetic energy in the moving cyclones of the
temperate latitudes.
The just described system of cells would only produce meridional winds, i.e.
winds from South to North or vice versa. The Earth's rotation is modifying this
meridional circulation system by the Coriolis force. Winds towards the poles get a
westerly component, winds towards the equator an easterly component. Therefore,
we mainly observe westerly winds at the ground in the Ferrel cellwhile we observe
easterly winds at the ground in the Hadley cell and the polar cell. The north
easterly winds near the ground of the Hadley cell are also known as the trade
winds. These global wind cells have a spatial scale of roughly 10,000 km. The
global wind system is modified by the temperature contrasts between the conti-
nents and the surrounding oceans and by large north-south orientated mountain
ranges, in particular those at the west coasts of the Americas. These modifications
have a spatial scale of some 1,000 km. Even smaller land-sea wind systems in
coastal areas may have an order of 100 km; mountain and valley wind systems can
be even smaller in the order of several tens of kilometres. All these wind systems
may be suitable for wind power generation.
While the trade winds and the winds in the polar cell exhibit quite some
regularity and mainly have seasonal variations, the winds in the Ferrel cell are
much more variable in space and time. Near-surface wind speeds in normal
cyclones can vary between calms and about 25 m/s within a few hours. Wind
speeds in strong hibernal storms of the temperate latitudes can reach about
35-40 m/s while wind speeds in subtropical hurricanes easily reach more than
50 m/s. Cut-off wind speeds of modern wind energy turbines are between 25 and
30 m/s. Thus strong storms in temperate latitudes may lead to phases where the
wind potential can no longer be used. These hibernal storms are most likely in
Northwestern Europe, Northeastern Canada, the Pacific coasts of Canada and
Alaska as well as the southern tips of South America, Africa and Australia.
Hurricanes are called typhoons in Southeast Asia and cyclones in India. The
occurrence of hurricanes can even threaten the stability of the construction of the
turbines, because they can come with wind speeds above those listed in the IEC
design standards. The hurricane risks have been investigated by Rose et al. (
2012
).
In particular, the planning of offshore wind parks in hurricane-threatened areas
needs special attention. According to the map of natural hazards published by the
reassurance company Munich Re, hurricane-prone areas are the southern parts of
the Pacific coasts and the Atlantic coasts of the United States and Central America,
Eastern India and Southeast Asia, Madagascar and the northern half of Australia.
There are very strong winds on even smaller scales such as thunderstorm
downbursts, whirlwinds and tornados, but their variability and destructive force is
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