Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 102

Table 5.1 Significant wave heights, H s for four wind direction sectors and stratification as
function of selected wind speed values at 100 m height from respective regression curves to those
shown in Fig. 5.8 (taken from Emeis and Türk 2009 )
Wind speed in m/s 25 30 35 40 Explained variance in %

w.r.t. wind speed Sector/stratification Significant wave height in m

North (290-40, usually unstable) 6.7 9.2 12.1 15.5 69.5
East (40-110), unstable 5.2 7.0 9.2 11.8 75.2
South (110-220), unstable 4.2 6.0 8.1 10.6 61.3
South (110-220), stable 3.4 4.8 6.5 8.5 53.0
West (220-290, usually stable) 4.1 5.3 6.6 8.1 56.6
East (40-110), stable 1.6 1.6 1.7 1.6 29.9
The entries in this table are ordered with decreasing H s for u = 40 m/s

definition of these sectors). The relatively low wave heights from the eastern and
especially from the southern wind direction sector have to be attributed to the
small fetches and therefore limited durations for which wind and waves can
interact in these two sectors. The minimum distance to the coast in the southern
sector is only about 50 km. Fetches are much longer for the western and the
northern sectors and reach or even exceed the spatial scales of atmospheric
depressions. Therefore, durations of 12-24 h can be assumed for these two sectors.
As waves are higher in the northern sector than in the western sector for the same
observed friction velocity, the waves from the northern sector must be older than
the waves from the western sector. Because wave periods were not easily available
the regression between the wave heights and the friction velocity has been
approximated by a quadratic expression in the four frames of Fig. 5.8 .
The wave heights in the different wind direction sectors have also been cor-
related to the wind speed in 100 m height. The results are given in Table 5.1 . Two
features become obvious: (1) The explained variance is somewhat lower when the
wave heights are correlated with the wind speed than with friction velocity, and (2)
for two sectors (east and south), the thermal stratification of the MABL becomes
important. Therefore, different regressions for stable and unstable stratification
have been listed in Table 1. In these two sectors, the land is still so close to the
measurement site that air considerably warmer (stable stratification) or colder
(unstable) can reach the FINO1 platform. The vertical mixing is not sufficient to
remove this stratification on the way from the coast to the platform. For the east
wind sector, this difference between stable and unstable stratification is depicted in
Fig. 5.9 . From this sectoral analysis, the highest wave heights at FINO1 in the
German Bight have to be expected from the northern sector.
The missing stratification dependence for westerly and northerly winds in
Table 5.1 indicates that stronger winds from the western and the northern sector at
FINO1 in the German Bight are linked to a limited range of possible thermal
stratifications. Northerly gale force winds at this site occur mainly during cold
air outbreaks on the rear side of cyclones moving east over Northern Europe,