Vertical Profiles Over Flat Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 53

Fig. 3.13 SODAR observations of a nocturnal low-level jet over Paris Airport Charles de Gaulle
in June 2005. Displayed are six consecutive half-hourly averaged wind profiles. The three curves
between 30 and 200 m are from ( 3.16 ) using L * = ?, 500 and 100 m (from left to right)

3.4.2 Low Level Jets

Over land, low-level jets are nocturnal maxima in the vertical wind speed profile
which form at the top of the nocturnal boundary layer. Typical heights are between
150 and 500 m above ground. Therefore, they have the ability to influence the
energy yield of modern wind turbines with hub heights of more than 100 m.
Figure 3.13 shows six subsequent half-hour mean profiles as an example.

3.4.2.1 Origin of Low Level Jets

The formation of low-level jets requires a temporal or spatial change in the thermal
stability of the atmosphere which leads to a sudden change between two different
equilibria of forces. The flow must transit from an unstable or neutral condition
where friction, pressure-gradient and Coriolis forces balance each other to a stable
condition where only pressure-gradient and Coriolis force balance each other (see
Fig. 3.14 ). The sudden disappearance of the retarding friction in the equilibrium of
forces leads to an inertial oscillation of the horizontal wind vector. Wind speed
shoots to much higher values and the increased wind speed leads to a stronger
Coriolis force which provokes a turning of the wind vector as well. The relevant
equations for this phenomenon have already been presented in Sect. 3.2.2 .
In the temporal domain this corresponds to a sudden change from an unstable
daytime convective boundary layer to a nocturnal stable boundary layer. This
requires clear skies in order to have rapid changes in thermal stratification but still