Spectral Analysis of a Perturbed Stable Boundary Layer

Turbulence data have been sampled on the Nansen Ice Sheet, a coastal area of Antarctica, in order to study how and at which scales topographic inhomogeneities and gravity waves propagation affect the turbulence structure of a stably stratified boundary layer. Spectral analysis of wind velocity components evidences an energy excess in the horizontal components due to a shift of the turbulence spectral maximum towards a lower frequency. The shift is proportional to the characteristic vertical length scale of topography. A sharp secondary energy maximum in the low-frequency subrange has been frequently evidenced by spectral analysis. It is prevalently observed in strongly stable stratification and the energy is often supplied by the gravity waves propagation, resulting from the univocal response of cospectral wave activity indicators. In weakly stable conditions, the secondary energy maximum is prevalently less pronounced and broadened over a larger frequency range. The cospectral wave indicators do not give a univocal response on the nature of the fluctuations, resulting from the non-linear interaction between waves and turbulence.