Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 109

Fig. 5.15 Variation of wind speed at 30, 40, 50, 60, 70. 80, 90, and 100 m height in m/s (upper
bundle of full curves from bottom to top, right axis), air temperature at 30, 40, 50, 70, and 100 m
in C(middle bundle of full curves, from bottom to top, leftmost axis), wind direction at 30, 50,
70, and 90 m in degrees (lower bundle of full curves, left axis), sea surface temperature in C
(horizontal line labelled ''water temperature''), surface pressure in hPa (upper straight line, left
axis), relative humidity in % (lower nearly straight line, left axis) and global radiation in W/m 2
(curve between 0 and 400, left axis) at FINO1 in the German Bight for the period October 26,
2005 12 UTC ? 1 to October 28, 2005 12 UTC ? 1
exceeds the value 0.14 given in the offshore IEC standard. For wind speeds above
15 m/s this even happens for the 10th percentile. For unstable conditions the
exponent rarely exceeds a value of 0.05.
Figure 5.15 gives an example how thermal stratification of the MABL directly
influences the vertical wind shear. The figure shows a record of 48 h duration.
Initially, air temperature is very close to sea surface temperature and the vertical
wind shear is small. After about 18 h, an episode of warm air advection starts,
which lasts for roughly 24 h. Immediately after the onset of the warm air advection
the vertical wind shear increases considerably, visible from the growing spread
between the wind speeds at different heights at the mast FINO1. In the afternoon of
the second day in the centre of the Figure, 100 m wind speed is about twice as large
as 30 m wind speed. This gives a shear of 8 m/s over a height interval of 70 m. This
large vertical shear disappears rapidly when the warm air advection ends at the end
of the displayed episode. This example shows that the air-water temperature dif-
ference is the decisive parameter which governs the vertical shear in the MABL. In
contrast to land surfaces, the change in static stability in the MABL is not coupled
to the diurnal radiative cycle but to passing weather systems (depressions).
Figure 5.16 gives an example of the monthly distribution of thermal stratifi-
cation in the MABL by displaying the spread between the potential temperatures at
the heights 30 and 100 m for October 2005. Potential temperatures are tempera-
tures corrected for the adiabatic temperature decrease with height. For neutral
stratification, potential temperature is constant with height. Potential temperatures
increase with height for stable stratification and decrease for unstable stratification.
During that month the average sea surface temperature at the mast FINO1 was