Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 105

20 consecutive values from the storm ''Britta'' have entered the analysis. This
duration of a storm event has been considered as a typical duration of an atmo-
spheric depression. As there are a lot of other (weaker) storms with comparable
durations also included in the analysis, the weight of ''Britta'' in this analysis can
be considered as fair.

5.1.4 Wave Age

The wave age is an important parameter which governs vertical profiles of wind
and turbulence in the constant flux layer of the MABL. The wave age describes the
type of interaction between the wind field and the waves. A distinction is made
between young and old waves. This distinction is necessary, because of the
delayed response of the wave field to the wind field and the hysteresis effects
which come with this delay. Young waves are wind-driven waves where the wind
speed is larger than the phase speed of the waves. This situation resembles the
usual situation over rough land surfaces and we expect principally a validity of
Monin-Obukhov similarity and the features described in Chap. 3 . Old waves are
waves which still exist after the wind force has already decreased again. Old waves
are often called swell when they come in from far away. These waves can be faster
than the near-surface wind, which essentially means that the waves drive the near-
surface wind and that upward turbulent momentum flux can occur in the surface
layer of the MABL. This behaviour can no longer be described by Monin-
Obukhov similarity, because this similarity approach assumes that the surface is a
momentum sink. The influence of wave age will become visible in some of the
results in the following subchapters.
Typically, wave age, c is defined as the ratio between the phase speed of the
waves, c ph and the friction velocity in the atmospheric surface layer:
c ¼ c ph
u

ð 5 : 11 Þ

The limit between young and old wave is roughly at 28, because the friction
velocity is in the order of 1/28 of the wind speed. Typical values for the wave age
range from 5 at high wind speeds to several hundred at very low wind speeds.
Figure 5.11 shows the relative frequency distribution for 2005 observed at the
FINO1 platform. The most frequent wave age is between 25 and 30, i.e. just
around the transit threshold between young and old waves. The average wave age
is much higher at 55.3 due to fewer but very large wave ages. Figure 5.12 displays
the relation between wind speed and wave age. As said above, young waves occur
at high wind speeds while old waves occur at very low wind speeds. This relation
is clearly depicted in this figure. Above a wind speed of about 18 m/s no more old
waves can be observed in the presented data set.