Proposal of an empirical velocity spectrum formula in low-wind speed conditions

Low-wind spectra are investigated through the analysis of sonic anemometer observations gathered in three experimental campaigns: the Urban Turbulence Project, the NOrthern hemisphere climate Processed land-surface EXperiment and the Graz experiment. From the comparison of the horizontal Eulerian experimental spectra computed in this work with those available in the literature it is shown that in low-wind conditions the spectra present an evident peak in the lower frequency range, that it is not accounted for in the classical formulations. This is a consequence of the oscillatory behaviour of the horizontal wind, and the peak frequency depends on the meandering time-scale of the horizontal wind components. In contrast with previous studies the meandering behaviour is observed in both stable and unstable conditions. A new formula for the low-wind velocity spectrum is proposed and evaluated.