Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 107

5.1.6 Annual and Diurnal Variations
The thermal properties of the sea surface are considerably different from those of a
land surface. Water has a much larger heat capacity than soil. Therefore, sea
surface temperature does not show diurnal temperature variations but mainly an
annual variation with a maximum in late summer and a minimum in late winter.
This annual variation is slightly modified by cold and warm air advections
occurring with moving atmospheric pressure systems on a temporal scale of a few
days. Thus, the strong diurnal variation of the vertical structure of the atmospheric
boundary layer, which is so familiar from land sites, is completely missing in the
marine boundary layer, except for coastal regions when the wind blows from the
land (see Sect. 5.6 ). We rather find a dominant annual variation. Unstable marine
boundary layers prevail in autumn and early winter and stable marine boundary
layers in spring and early summer. This seasonal pattern comes from the generally
larger thermal inertia of the sea water which leads to a time shift in the order of
1 month of the annual temperature variation of the water with respect to the
atmospheric annual temperature variation. Therefore, we find cooler air masses
over the warmer sea water in autumn while we have warmer air masses over the
cool oceans in spring.
5.2 Vertical Profiles
Usually, hub heights in offshore wind parks are above the often quite shallow
constant flux or surface layer (see Fig. 5.1 ). Hub heights are rather in the Ekman
layer of the MABL where we find only a slight wind speed increase and a slight
turning of the wind direction with height. Therefore, a vertical extrapolation of the
wind profiles using the power law ( 3.22 ) instead of the stability-dependent loga-
rithmic law ( 3.16 ) is suitable. It is demonstrated in Sect. 3.1.3 above that for very
smooth surfaces such as the sea surface the difference between the logarithmic
profile and the power law profile are small.
Figure 5.13 shows the frequency distribution for the power law profile exponent
a from the mast FINO1 in the German Bight. These exponents have been derived
from 10 min-averaged wind profiles at the height range between 40 and 90 m
taking 40 m as reference height. The most frequent value is 0.03, the mean value is
0.10. These values are much lower than those over land (see Fig. 3.4 ). The
exponent depends considerably on wind speed and thermal stratification
(Fig. 5.14 ). The increase with wind speed is absent over land where the exponent
approaches a constant value for very high wind speeds. The offshore power law
exponent increases with growing wind speed, because the waves grow and the sea
surface gets rougher with increasing wind speed. In the same manner as over land,