One of such cycles happens at nighttime in periods of large-scale flow dominated by a high-pressure field located over the Atlantic Ocean. Such a system drives winds with a northerly component towards the site. In the first hours after the evening transition, these winds come typically from the ocean, keeping moderate levels of static stability and continuous turbulence. As the night progresses, the mean wind direction slightly shifts counterclockwise, causing the advection of terrestrial, rather than maritime air, towards the site. Such a transition typically happens between 0000 and 0200 LST at the site, largely enhancing local stability, causing turbulence decay and mean wind speed decrease.
Therefore, along a typical night with this large-scale condition, this site experiences a few hours of weakly stable stable boundary layer, followed by subsequent hours under very stable stratification, making it ideal to studythe transition between these two regimes. The purpose of the present study is to use this dataset to analyse in detail this transition, both temporally and vertically.
A total of 98 nights between August 2016 and July 2017 are analysed. First, a few detailed case studies are shown, and then typical composites of the variables are presented. Temporal evolutions of quantities such as the thermal gradient, mean wind speed, turbulent kinetic energy (TKE), heat flux, graident and flux Richardson numbers and the terms of the TKE budget are presented for the different vertical levels. Special emphasis is given to the interplay between them, and how they evolve vertically during each of the regimes and, in particular, at the transition between regimes. In agreement with recent studies, it is shown that simple variables such as the mean wind speed, exert stronger control on the stable boundary layer regime than turbulence-related ones, such as the flux Richardson number.
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