Classification of Low-Level Jets at Cabauw

Low-level wind maxima are frequently occurring phenomena in the stable nocturnal boundary layer. After sunset the flow can become decoupled as a result of surface cooling. Above the decoupled surface layer the horizontal balance of forces is disturbed, the flow begins to accelerate and a low-level jet (LLJ) is forming.

Vertical shear below LLJs is likely to affect the turbulent exchange between the surface and the atmosphere. This is especially true if the LLJ is close to the surface. Furthermore, LLJs can play a significant role in the transport of pollutants. From the perspective of wind energy and aviation, knowledge of LLJ characteristics is also very useful.

The goal of this study is to produce relevant statistics on the behavior of nocturnal LLJs, which can be used to evaluate the bulk-performance of atmospheric models in stable conditions. Using long-term mast and wind-profiler observations from the Cabauw-site, we classify vertical profiles of wind speed to wind forcing and nocturnal cooling. In general, we find that the frequency of occurrence of LLJs increases with decreasing wind forcing and with increasing cooling. The turning of the wind vector between the jet nose and the surface seems to be rather insensitive to the wind forcing, but becomes larger for increasing cooling. The average height of the jets increases with stronger wind forcing and with weaker cooling.