Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 99

Fig. 5.6 Wind speed at 80 m (full line) and friction velocity, u * ,at40m(dotted) above mean
sea level together with the wave age (dashed line), the air-sea temperature difference (dashed-
double dotted line), the measured hourly wave height (black squares) and the calculated wave
height from Eq. ( 5.10 )(thick full line) at FINO1 in the German Bight during the violent storm
''Erwin'' on January 8, 2005 (x-axis gives time in hours). All data except measured wave heights
are 10 min mean data. u * refers to the right-hand axis, all other variables to the left-hand axis

the significant wave height, H s , for growing waves independent of the fetch if the
wave period T is known:

H s ¼ 0 : 062

p

u gT 3 ð 5 : 10 Þ

with g the acceleration of gravity. Maat et al. ( 1991 ) give a somewhat smaller
value for the constant in Eq. ( 5.10 ), namely 0.051.
Figure 5.5 shows a broad scatter of wave height data with wind speed. The two
curves in Fig. 5.5 show the ''trajectories'' for the storms ''Britta'' and ''Erwin'' in
the wave height-velocity phase space. These trajectories demonstrate that a larger
part of the scatter in Fig. 5.5 happens with the evolution of the wave height-
velocity relation during the passage of single low-pressure systems. The curves are
plotted through consecutive hourly data points. The full curve for the All Saints
Day storm ''Britta'' is 21 h long from October 31, 2006, 1700 h to November 1,
1400 h. The second curve for the gale force storm ''Erwin'' covers a time span of
23 h from January 7, 2005, 2300 h to January 8, 2200 h. This is analysed in more
detail in Figs. 5.6 and 5.7 . During ''Britta'' (Fig. 5.7 ), the peak H s is reached about
1 h after the peak wind speed of 31 m/s; during ''Erwin'' (Fig. 5.6 ), it is reached
about 5 h after the main wind speed peak of 32 m/s. The two curves for ''Britta''
and ''Erwin'' differ considerably. The curve for ''Britta'' lies at much higher wave
heights than the curve for ''Erwin'', although the peak wind speeds are quite
similar. Looking at the wind direction and the air-sea temperature difference, it
turns out that the major difference in atmospheric conditions is that during
''Britta'', cold air advection was prevailing with northerly winds, and during
''Erwin'', warm air advection with westerly winds.