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ABSTRACT
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 7-yr 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, are typically situated at 140-260 m above ground level, and have a speed of 6-10 m s^sup -1^. Development of a substantial LLJ is most likely to occur for moderate geostrophic forcing and a high radiative cooling. A comparison with the 40-yr ECMWF Re-Analysis (ERA-40) is added to illustrate how the results can be used to evaluate the performance of atmospheric models.
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1. Introduction
Low-level jets (LLJs) are frequently observed phenomena in the nocturnal atmosphere in many parts of the world. They are characterized by a maximum in the wind speed profile, which is typically situated 100-500 m above the earth's surface. In the literature, many studies can be found on the development and the characteristics of LLJs (e.g., Bonner 1968; Garratt 1985; Kraus et al. 1985; Whiteman et al. 1997; Andreas et al. 2000; Banta et al. 2002; Song et al. 2005).
Knowledge of the characteristics of the LLT is relevant for aviation, wind energy applications (Storm et al. 2008), the transport of pollutants (Beyrich 1994), and other atmospheric constituents like ozone (Banta et al. 1998) and carbon dioxide (Mathieu et al. 2005; Karipot et al. 2006). Furthermore, the strong shear below the jet can influence the turbulent exchange between the surface and the atmosphere (Banta et al. 2006; Conangla and Cuxart 2006). Recent wind tunnel experiments showed that LLJ-generated shear may cause intermittent bursts of turbulence in the lower part of the stable boundary layer (SBL)...