Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 108

Fig. 5.13 Frequency distribution of the power law profile exponent a in percent [see Eq. ( 3.22 ) ]
for a reference height z r = 40 m at FINO1 in the German Bight for the period September 2003 to
August 2007 for wind speeds higher than 5 m/s at 100 m. Bin width is 0.01. The leftmost column
summarizes all occurrences with even more negative values
Fig. 5.14 Power law profile
exponent a [as in Fig. 5.13 )]
as function of wind speed at
40 m height at FINO1 in the
German Bight for neutral
stratification (bold lines
showing the 10th percentile,
the mean and the 90th
percentile), unstable
stratification (dashed lines)
and stable stratification
(dotted lines)
the exponent also grows with increasing thermal stability, because the vertical
shear of horizontal wind speed increases in stable conditions due to suppressed
vertical turbulent mixing.
While the mean value for the power law profile exponent for neutral conditions
is below the value 0.14 which is assumed in the normal wind profile model (NWP)
of the standard IEC 61400-3 (2006) for offshore wind turbines, it can happen that
the exponent is sometimes above this value (see Fig. 5.14 ). At mean wind speeds
of 12-13 m/s the 90th percentile for the power law exponent is even above the
onshore value of 0.20 given in the IEC standard. The 90th percentile decreases
again with higher wind speeds while the mean value for this exponent still
increases. This is because the distribution of this exponent becomes much nar-
rower with increasing wind speed. For stable stratification the exponent regularly