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Figure 81: Efficiency of a large-scale synoptic weather pattern (Grosswetterlage) to form a
low-level jet. From two years of sodar data for Hannover, Germany.
Figure 82: Correlation between the height of the core of a low-level jet and its maximum wind
speed over Northern Germany (from two years of sodar measurements).
formation probability for Northern Germany for each of the weather types. For other areas
the investigation has to be repeated with local low-level jet data.
Theheightofthecoreofalow-leveljetanditsmaximumwindspeedseemstobecorrelated.
Figure 82 shows a correlation analysis from two years of sodar measurements in Northern
Germany. Weaker jet cores may appear at heights between 150 m and 200 m above ground
while stronger events (20 m s −1 core wind speed) usually appear at about 400 m with
considerable scatter. Typical wind shear values for the layer between the surface and 160 m
above ground vary between 0.04 and 0.10 s −1 . An analysis of daily wind speed amplitudes
at 160 m above ground from these sodar data shows that amplitudes of up to 14 m s −1 are
possible due to low-level jet events.
6.5 Summary
Wind resources depend on the large-scale weather conditions as well as on the local vertical
structure of the atmospheric boundary layer. Ground-based remote sensing is now a viable
technique to monitorthe vertical structure of the atmospheric boundary layer. Three different
techniques are presently available: acoustic sounding (sodars), optical sounding (wind lidars
and ceilometers) and the combination of acoustic and electro-magnetic sounding (RASS).
138 DTU Wind Energy-E-Report-0029(EN)