Spectral characteristics of waves and turbulence in a stable boundary layer

Turbulence measurements have been performed in Antarctica on the Nansen Ice Sheet, a permanently frozen branch of the Ross Sea bordered by mountains rapidly rising toward the plateau. Spectral analysis of wind velocity and temperature fluctuations has been performed by Fast Fourier Transforms to investigate how and at what scales the stable boundary layer is perturbed by the orographic forcing and by the propagation of gravity waves. Taking into account reference spectra obtained over a uniform and flat surface (Kaimal et al. 1972), it has been observed that: - orographic complexities, as well gravity waves, leave unaffected the inertial subrange; - horizontal velocity components, due to the orographic forcing, display an energy increment in the mid and low frequency subranges; this effect is less marked in the vertical component; - in complex orography the maximum of turbulent energy is shifted towards lower frequencies and the corresponding wavelength, being roughly equal to the height of the slope, is the scaling parameter of the up-stream flow fall; - gravity waves propagation in a strongly stable boundary layer results in a secondary energy maximum displaying a spectral shape predicted by similarity hypotheses. The wave contribute is confirmed by wave activity indicators; - often in mid stable or near neutral conditions a single or multiple, quite flat and broaden, energy maxima are generated by strong non-linear interactions of waves with turbulence. Wave indicators are not able to give an univocal response because, in the energy transfer process, waves loose their peculiar properties.