Geoscience Reference
In-Depth Information
Fig. 5.7 As Fig. 5.6 , but for
violent storm ''Britta'' on
October 31/November 1,
2006. Between 11 p.m. and
midnight and between 11
a.m. and noon, some data are
missing
Figures 5.6 (Erwin) and 5.7 (Britta) analyse the relation between the local state
of the atmosphere and the wave height in these two storm events in some more
detail also using some of the 10 Hz data from the sonic anemometers at 40 and
80 m. The atmospheric conditions are described by the wind speed, u, at 80 m and
the friction velocity, u * , at 40 m height. In addition, the temperature difference
between the air temperature at 40 m height and the sea surface temperature, the
measured hourly values of the significant wave height, H s , the calculated signifi-
cant wave heights using Eq. 5.10 and the wave age (i.e. the phase speed of the
waves over the friction velocity, see also next subchapter) are given. Figure 5.6
shows a slightly stably stratified boundary layer due to warm air advection over
colder water (see temperature difference). The mean ratio, z/L * , is only about +0.03
(about 0.06 in the first half of the displayed period and nearly 0.0 in the second
half, not shown in the figure). The vertical wind shear between 40 and 80 m is
about 3 m/s and decreases from 1 to 2 m/s after the occurrence of the peak wind
velocity. The peak wind speed is accompanied by a maximum in the friction
velocity (1.4 m/s) and a minimum in the wave age (about 11). The greatest
increase in the wave height coincides with the highest values of the friction
velocity. The peak wave height is observed 5 h after the peak wind speed at a wave
age of about 18. For the whole period shown in Fig. 5.6 , the wave age remains
below 24, i.e. we have a wind-driven sea all the time. Figure 5.7 shows an unstably
stratified boundary layer during cold air advection of warmer water. The mean
ratio, z/L * , is only about -0.06 (between midnight and 11 a.m.). Negative peak
values of z/L* of up to -0.79 occur between 2 and 4 p.m. (not shown in the
figure). There is nearly no vertical wind shear in the layer between 40 and 80 m.
Also, the friction velocity is nearly constant with height. Thus, this layer seems to
be a constant flux layer (Fig. 5.1 ). Again, the largest increase in wave height
coincides with the phase of the highest friction velocity (again 1.4 m/s). The peak
wave height is observed about 1 h after the peak wind speed at a wave age of about
11-12. Like in Fig. 5.6 , the wave age shows a minimum associated with the
maximum in the friction velocity, u * , but in contrast to the situation during storm
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