Regimes and interactions of thermally driven flows during MATERHORN experiment

Nocturnal downslope flows are a key component of the diurnal mountain wind system. They consist of shallow, cold-air layers descending along mountain slopes. Oscillations in their velocity and temperature have been reported and linked both to internal mechanisms and external forcings, resulting in non-stationary flow dynamics. This thesis investigates the nocturnal evolution of downslope flows under non-stationary conditions, using case studies from the MATERHORN campaign. These showed an initial undisturbed downslope regime after sunset, followed by a perturbed phase with transient events and intrusions of external air masses. The aim was to identify oscillatory modes, their periods and persistence in wind observations using the Continuous Wavelet Transform (CWT), a methodology not previously applied in this context. The CWT, based on repeated convolutions of the signal with localized basis functions called wavelets, enables the tracking of dominant frequencies in time-varying signals. This thesis therefore pursues two complementary objectives. First, it evaluates the use of the CWT for analyzing non-stationary features in wind measurements, testing its ability to identify dominant frequencies, and track their evolution over time. Second, it provides a physical interpretation of the observed phenomena, linking spectral signatures to underlying dynamical processes. Wavelet analysis revealed multi-scale, time- and space-dependent dynamics: persistent oscillations in undisturbed phases, and stronger, transient modes during flow interactions. Short-lived events, consisting of external wind intrusions, were highlighted by the wavelet time–frequency representation. These findings underline the complex, non-stationary nature of nocturnal downslope flows and demonstrated the effectiveness of wavelet techniques in capturing their transient dynamics, providing an enriched view of the processes shaping the nocturnal evolution of downslope flows.