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Fig. 6.8 Frequency distribution of the wake length of an indefinitely broad wind park in km
using the stability data from Fig.
6.6
. Bin width is 1 km. Wake length has been defined as the
distance where 95 % of the original power yield is reached again
coast, the probability is estimated that such a wind park will be affected by
waterspouts. This estimation does not look for the probability that a single wind
turbine is hit by the vortex centre, i.e. the probability of a mathematical point being
hit (Thom
1963
) is not investigated. Due to the horizontal wind shear across the
vortex' core and mantle regions, even a near miss by a waterspout may be
hazardous for a wind turbine. In addition, it is presently unclear if the small-scale
wind field in a wind park altered by the wind turbine wakes themselves
(Christiansen and Hasager
2005
) may actually increase the likelihood of a hit once
a waterspout enters an array of wind turbines. Therefore, the recurrence time of a
waterspout anywhere within the wind park instead of at an individual wind turbine
site is analysed.
Taking the waterspout incidence presently known for the German North Sea
coast [which is about one tornado per 10,000 km
2
per year, based on estimates by
Koschmieder (
1946
) or Dotzek (
2003
)], one can expect one tornado in an offshore
wind park once within one hundred years. This includes the assumption that
waterspouts occur homogeneously over the German Bight area. If using the upper
limit of Koschmieder's estimate, which is two waterspouts per 10,000 km
2
per
year, this recurrence time reduces to 50 years for a single wind park.
While this still seems to be a long interval, one has to take into account that the
total area of approved or actual off-shore wind parks in the German Bight is
already 648 km
2
in 2010 (Source: German Federal Maritime and Hydrographic
Agency; Bundesamt für Seeschifffahrt und Hydrographie), leading to a recurrence
interval of less than eight years for any wind park to be hit by waterspouts in a
given year, based on Koschmieder's incidence estimate of two waterspouts per
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