Valley Breezes within a valley in Pyrenees: Vertical and Horizontal structure and its Evolution Over the Annual Cycle Through Observations

Nocturnal downvalley flows are analyzed in a valley in southern France, near the Pyrenees. Three meteorological stations were installed at different locations strategically chosen in the valley within the framework of the LATMOS-i1 and WINDABL2 projects and in collaboration with the French MOSAI3 project. In addition to the near-surface measurements, including turbulent parameters, several radio soundings during down-valley flow cases were launched during the night to characterize the vertical dynamic and thermal structure of these winds.

Near the surface, downvalley winds are characterized by southerly (from the Pyrenees) and progressively increasing winds, which produce higher values of turbulent parameters than those observed during the day. In addition, the vertical structure of the flow shows significant variations during the night, influenced by a complex interaction between the synoptic conditions and the surface processes. On days with strong synoptic forcing, typically from the west in the study region, downvalley flow formation is completely inhibited. However, on days with 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 within the valley in a shallower layer. An analysis of the atmospheric stability using the bulk Richardson number at different layers is also presented. An emphasis is placed on differentiating those layers with higher static/dynamic stability to distinguish whether the turbulence is related to ground-induced thermal effects or dynamically driven by the wind. The analysis is completed by simulations with the WRF model to evaluate its ability to reproduce these events and to point out its shortcomings in order to improve the prediction of these events.

The availability of one year of surface data allows an analysis of the evolution of the structure of these flows over the annual cycle. For this purpose, an algorithm for detecting breeze events and different statistics will be used for their analysis.

This study highlights the complexity of observational studies attempting to differentiate the factors influencing nocturnal downvalley flow, and emphasizes the need to consider both synoptic conditions and surface processes, including the important role played by local topography. 

(1) LATMOS-i project (Land-ATMOSphere interactions in a changing environment: How do they impact on atmospheric-boundary-layer processes at the meso, sub-meso and local scales in mountainous and coastal areas?) (PID2020-115321RB-I00, funded by MCIN/AEI/ 10.13039/501100011033).

(2) WINDABL project (PR2022-055). Project to impulse the career of young researchers funded by the University of Cádiz (Spain) (Plan Propio).

(3) MOSAI project (Model and Observation for Surface-Atmosphere Interactions, https://mosai.aeris-data.fr/).