Publishers version - DTU Orbit

Figure 76: Combined soundingwith a sodar(black squares and red asterisks) and a ceilometer
(blue triangles) giving a complete view of the diurnal variation of the vertical structure of the
ABL.
Figure 77: Example of a sodar observation of a nocturnal low-level jet.
6.4 Low-level jets
Sometimes vertical wind profiles exhibit secondary maxima in heights which are relevant
for today’s large wind energy converters, because the vertical wind shear underneath these
maxima may be enhanced considerably. Figure 77 shows an example for a nocturnal low-level
jet over land.
6.4.1 Formation
The formation of low-level jets requires a temporal or spatial change in the thermal stability
of the atmospheric boundary layer 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. 78). The sudden disappear-
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