Flow perturbation due to the orographic forcing at the top of a steep ridge has been investigated. Spectral and wavelet analyses of longitudinal and vertical wind velocity components highlight the presence of low-frequency perturbations produced by topography. Wavelet kurtosis reveals the intermittent character of these perturbations. The application of a multi-resolution data filter allows the detection of the topographically forced structures and the consequent statistical characterisation. The detected topographically forced structures remain active only for a 15% of the time on average. Nevertheless, they appear very efficient in the momentum transport, accounting for about 50% of the downward momentum flux produced in the low frequency range. Quadrant analysis highlights how topographical forcing produces the weakening of ejections and the strengthening of sweeps at the top of the ridge.

Wavelet and quadrant analyses were applied to turbulent velocity data in order to investigate the transition from the anisotropy of energy-containing eddies to the isotropy of the inertial subrange scales. The quadrant analysis of the wavelet coefficients of longitudinal and vertical velocity components allows the evaluation of the velocity structure functions and the momentum cospectrum as a function of the separation distance and of the quadrants. In an isotropic condition the contribution both of ejections and sweeps (even quadrants), and both of reflections and deflections (odd quadrants), has to be equal. The analysis has shown that in neutrally stratified conditions the transition to isotropy occurs in a frequency range (0.2 < r/z < 3) usually referred to as internal to the inertial subrange (r is separation distance, z is height). In the transition region, as in the isotropic region, the velocity structure functions still agree with the 1941 and 1962 Kolmogorov theories; but on the other hand the structure functions of the even and odd quadrants are fitted by power laws of different slopes in the transition region. The proposed analysis allows the investigation within the transition region of the different dynamical structure in the energy transfer from the energy-containing scales to the isotropic scales.

Wavelet analysis of longitudinal and vertical velocity components has been performed to investigate local isotropy conditions in the inertial subrange, Analysed turbulence data have been collected in Antarctica. A conditional sampling have been applied to data in order to eliminate intermittency. The conditioned structure functions have been computed to identify the inertial subrange extension through the validation of the K41 similarity relations. The quadrant analysis of the momentum cospectrum allows to locate the low-frequency limit of the isotropic region and to identify the anisotropic eddy motions within the inertial subrange; i.e, the scales associated to the transition from the anisotropy of the energy-containing scales to the isotropy of the small scales. The anisotropy in the inertial subrange can be associated to the direct interaction between large-scale and small-scale motion.

In order to study the structure of the momentum flux in the inertial subrange, wavelet transforms have been performed on wind velocity time series. A conditional sampling has been utilised for separating intermittent contributions. A quadrant analysis technique has been applied to the intermittent and non-intermittent momentum fluxes. Their dependence on length scales, quadrants and stability has been investigated.

Turbulence measurements by ultrasonic anemometers have been performed at three levels on the Nansen Ice Sheet, Antarctica, over a gently sloping surface. Wind velocity profiles have been computed for down- and up-slope flows, at different stability conditions. The deviations of experimental turbulent profiles from the Monin Obukhov similarity theory indicates the effects of the sloping surface, as a function of the atmospheric stratification.