Observations of submeso motions and intermittent turbulent mixing across a low level jet with a 132-m tower

The coexistence of wavelike submeso motions and anisotropic intermittent turbulence in a night time stable boundary layer is investigated. Submeso motions of different characteristics and amplitudes interact with each other. These interactions may lead to intermittent turbulence production that alters the turbulent structure of the stable boundary layer. On the other hand, the production and transfer of turbulence affect the delicate balance of submeso motions. In this work sonic anemometer data collected at eleven levels in south-eastern Brazil have been used to study a case of a nocturnal boundary layer in a coastal site. The absence of forcing at the synoptic scale allows the development of a breeze circulation on which a low level jet of moderate intensity (4 m s-1) and low height (about 50 m) takes place. The jet evolution is coupled with dirty waves, while its full development is associated with gravity waves driven by a strong vertical temperature gradient. The layer centred at the low-level jet nose is characterized by horizontal meandering and very weak turbulence intensity. The air far below and above the low-level jet maximum experiences bursts of significant increase of the turbulence intensity, showing a three-layer structure. The oscillations of the wind velocity horizontal components exhibit the same frequency of the temperature oscillations, suggesting that the presence of an adequate temperature horizontal gradient is one of the fundamental driver of the meandering phenomenon. The considered night has been studied by means of the Eulerian auto-correlation functions for the detection of the meandering hours and their oscillation time-scales and by means of the continuous Morlet wavelet function for the detection of the gravity waves and the characterization of their spatial time scales and temporal evolution.