Nocturnal Downvalley Flow Characterization in a Valley Near the Pyrenees (France)

This study investigates nocturnal downvalley flows in a valley located in southern France, near the North face of the Pyrenees. Three meteorological stations were strategically installed at different sites across the valley, collecting a near year-long record of near-surface observations. These data enable a comprehensive examination of how these flows organize and evolve throughout the annual cycle.

Downvalley flow events were identified using a breeze detection algorithm (Arrillaga et al., 2018; Román-Cascón et al., 2019). Afterward, each event was characterized by its onset, peak intensity, and duration, with particular attention to the synoptic conditions favoring its development. In addition, sets of events occurring under different moderate synoptic conditions will be identified and examined to assess how such intermediate atmospheric forcing influences the final characteristics of the flows. This comparative perspective aims to provide a more holistic understanding of the interplay between local thermally driven dynamics and larger-scale atmospheric factors, offering valuable insights into nocturnal drainage flow processes in complex terrain.

An analysis was also performed to differentiate various turbulence regimes, employing HOckey STick (HOST) analysis (Sun et al., 2025)  to further evaluate how turbulence interacts with flow dynamics. Overall, these findings contribute to a deeper understanding of the mechanisms that govern  thermally driven flows in complex terrain throughout an entire annual cycle.

Furthermore, the research plan includes comparing the behavior of these downvalley flows strictly within the valley to those that manage to extend onto the adjacent plateau, in order to determine whether there are differences in persistence, spatial extent, or governing mechanisms when the flows exit the confines of complex terrain.