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Fig. 5.8 Significant wave height in m versus friction velocity, u
*
, at 40 m (hourly data) for
westerly (upper left), southerly (upper right), easterly (lower left), and northerly (lower right)
winds at FINO1 in the German Bight
''Erwin'', the wave age is changing from young to older waves about 10 h after the
passage of the peak wind speed at FINO1. In general, we find in both cases an
anticorrelation between friction velocity and wave age.
Because the values for the stability parameter z/L
*
for the two cases are so close
together, the values for the friction velocity are very similar in both cases. The
calculated wave heights from Eq.
5.10
in Figs.
5.6
and
5.7
have been produced by
choosing 0.056 as a value for the constant in Eq.
5.10
because this value gives the
best fit. This value turns out to be between the ones proposed by Maat et al. (
1991
)
and by Toba (
1978
). It becomes obvious that the calculated wave height is above
the measured one as long as the wave height is increasing due to the shear stress
exerted by the atmosphere on the sea surface. The periods of overestimation from
(
5.10
) coincide with wave ages close to 12 or even lower. After having reached the
peak wave height, the calculated wave height is slightly lower than the measured
one in Fig.
5.7
. This systematic deviation after the peak wave height—which
becomes especially notable in Fig.
5.7
for wave ages over 24 (equilibrium to old
waves)—is meaningful because Toba's relation has been derived for growing
waves only. On the other hand, the overall comparison between measured and
calculated wave heights turns out quite well and therefore provides an independent
confirmation of the values for the friction velocity, u
*
, determined from the sonic
anemometer measurements.
It is not meaningful to derive a relation between wave height and wind speed
from the data plotted in Fig.
5.5
due to the large scatter. Therefore, Fig.
5.8
presents the data separately for four different wind sectors (see Table
5.1
for exact
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