A climatology of nocturnal low-level jets at Cabauw

A climatology of nocturnal low-level jets (LLJs) is presented for the topographically flat measurement site at Cabauw, The Netherlands. LLJ characteristics
are derived from a seven year half-hourly database of wind speed profiles, obtained from the 200 m mast and a wind profiler. Many LLJs at Cabauw originate from an inertial oscillation, which develops after sunset in a layer decoupled from the surface by stable stratification. The data are classified to different types of stable boundary layers by using the geostrophic wind speed and the isothermal net radiative cooling as classification
parameters. For each of these classes LLJ characteristics like frequency of occurrence,
height above ground level, and the turning of the wind vector across the boundary layer are determined. It is found that LLJs occur in about 20% of the nights, that they are typically situated at 140 to 260 m above ground level, and have a speed of 6 to 10 m s-1. Development of a substantial LLJ is most likely to occur for moderate geostrophic forcing and a high radiative cooling. A comparison with model reanalysis (ERA40) is added to illustrate how the results can be used to evaluate the performance of atmospheric models.