During the FINTUREX experiment at the Neumayer station in 1994 meteorological
data like wind profile, temperature, humidity, and irradiation were measured
in order to determine the energy exchange and their parametrization over
an antarctic ice shelf.
Since the measurements were performed in the stable stratified boundary layer, our data are composed of waves, turbulence and intermittent bursts. This means that the determination of the energy exchange requires a powerful tool that makes it possible to separate (filter) the different contributions (waves and turbulence) as well as to analyze the localized intermittent bursts.
Due to the lack of locality in the time domain, the Fourier transformation (FT) has been increasingly substituted by the wavelet transformation (WT) for the analysis of turbulent data in the last two decades. Because of their localization in time as well as in frequency domain, the WT is the ideal candidate for the analysis of such data. Beside the possibility of calculating global spectra according to the FT, also local quantities like local energy spectra and a local intermittency measure can be determined. Additionally, due to the existence of an inverse transformation, the WT is perfectly suited for filtering.
Because of the variability of our data, it is desirable to find a typification that describes the main characteristics of a particular timetrace. We present a method that allows such kind of classification. Since the wavelet spectra of our data exhibit a well developed local minimum at the Brunt-Vaisälä frequency fBV (i.e. the maximum frequency of waves), the ratio of the energy content of the spectra above and below fBV can be used for a rough classification of the timeseries in respect to the contributions of waves and turbulence.
Another most challenging question has been the nonlinear interaction between the waves and the turbulent flow. We present a new method of direct visualization of the energy transfer from long scales (waves) to the smaller scales of turbulence (Richardson cascade) by means of the first moment (i. e. center of mass) of the local energy spectra and its behavior in time. We are quite confident that on the basis of this analyzing method the energy transfer rate between the generations of eddies can be determined. For a comparison data from a cylinder wake in a wind channel are also analyzed.
The 13th Symposium on Boundary Layers and Turbulence