Physics of Wind Parks - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 143

Fig. 6.5 Normalised reduction of available wind power at hub height behind an indefinitely
large wind park as function of the distance from the rear side of the park. Above: as function of
surface roughness (z0 = 0.0001 m: very smooth sea surface, 0.001 m: rough sea surface, 0.1 m:
smooth land surface, 1.0 m: rough land surface) with neutral stability. Below: as function of
atmospheric instability (h/L* =-1: strong instability, 0: neutral stability, ? 1: stable stratifi-
cation) for a smooth sea surface
recovered to 95 % of their original value upstream of the park. The most frequent
wake length is 11 km, the median is nearly 14 km and the weighted mean is
17.7 km. The 90th percentile is observed at 31 km and the 95th percentile at
37 km.
6.5 Risks that a Tornado Hits a Wind Park
Tornadoes are a risk for wind turbines. The weakest (F1) tornadoes have a wind
speed of 32-50 m/s, while F2 tornadoes reach 70 m/s which is well above the
survival speed of wind turbines. But even if the peak wind speed is below the