Observational Study of Valley Breezes in Heterogeneous Terrain: Vertical and horizontal characterization in the Aure Valley (Pyrenees)

This research focuses on the observational analysis of the nocturnal downvalley flows within a valley in southern France, near Pyrenees.  Three meteorological stations strategically positioned throughout the valley were installed at different locations within the frame of the LATMOS-i* and WINDABL** projects and in collaboration with the french project MOSAI***. In addition to the measurements near the surface, several radiosoundings were launched during the nights with downvalley flow in order to characterize the vertical structure of these winds. Near the surface, nights with downvalley are characterized by southerly (from the Pyrenees) and progressively increasing winds that produce higher values of turbulent parameters than those observed during daytime when weak synoptic conditions are present.  Moreover, the vertical structure of downvalley flow presents significant variations throughout the night, influenced by a complex interaction between the synoptic conditions and the surface processes. Days characterized by strong synoptic forcing, typically from the west in the study region, completely inhibit the downvalley flow formation. However, on days with even moderate synoptic forcing, the north-south orientation of the valley, coupled with the presence of mountains, seems to act as a shield against synoptic winds, allowing the nocturnal downvalley flow to form inside the valley. An analysis of the atmospheric stability using bulk Richardson number at different layers will also be presented. A key focus is to differentiate those layers with higher static/dynamic stability to discern whether turbulence originates from ground-induced thermal effects or dynamically driven by wind. This study highlights the complexity of observational studies trying to differentiate the factors influencing the nocturnal downvalley flows behavior, emphasizing the need to consider both synoptic conditions and surface processes, including the significant roles played by local topography.