A wavelet analysis of low-wind-speed submeso motions in a nocturnal boundary layer

In low-wind regimes (wind speed less than 1.5 m s−1) the nocturnal boundary layer is still inadequately understood. In such conditions, turbulence is weak and often intermittent, whereas dynamics and total fluxes are often driven by submeso motions. As a consequence, the momentum, mass and energy transfers are poorly represented by dispersion models. In low-wind conditions, an important fraction of submeso motions is represented by meandering modes, which can be detected through the Eulerian autocorrelation functions of horizontal wind components and temperature. Such an approach has been proven to be reliable. However, a deeper insight could be useful for understanding the phenomenon, especially when complex or multiple mesoscale motions simultaneously develop. In this work, Eulerian autocorrelation functions and the Morlet continuous wavelet transform were used to investigate an 8 h nocturnal period characterized by the coexistence of horizontal meandering and vertical oscillations, rarely observed elsewhere. This ‘nice’ episode represents a good case-study to compare the two used methodologies and to investigate the phenomenology of the two submeso phenomena simultaneously occurring in a low-wind-speed regime. The two methodologies identified the same time-scale for the detected meandering structures over the whole period. Moreover, the wavelet analysis: (i) was able to discriminate horizontal meandering and gravity waves, which simultaneously developed in the second part of the analysed period; (ii) showed that, in the investigated case, the horizontal meandering was not triggered by gravity waves which appeared later in the night; (iii) highlighted how both gravity waves and meandering can contribute to an increase of the vertical turbulent energy and fluxes, confirming the crucial role of submeso structures in the turbulence production during low-wind regimes in stable conditions.