Nocturnal Valley Winds in the Aure Valley (France): Analysis of Case Studies using Radiosoundings and High-Resolution WRF simulations

Nocturnal valley winds (NVWs) strongly modulate atmospheric stability, turbulence, and scalar transport in complex terrain. However,  their sensitivity to synoptic forcing and their representation in numerical models remain uncertain and extremely site dependent. 

In this work, we analyse an 11-month observational dataset, including several stations deployed at distinct, pre-selected locations along the Aure valley (near central French Pyrenees). These data provide complementary surface wind, radiation and turbulence observations. The valley is oriented north–south , opened to the north onto the Lannemezan Plateau, and its regional synoptic climatology is mainly dominated by westerlies (largely perpendicular to the valley axis).

From the observations, representative NVW cases are selected using a detection algorithm based on local and synoptic filters applied at each station, allowing the selection of events under contrasting large-scale conditions. NVWs are found even under moderate synoptic forcing (700 hPa winds higher than 12 m s⁻¹), consistent with the role of orographic shielding. Particular attention is given to moderate westerly situations (perpendicular to the valley main axis) in which the synoptic flow and the NVW coexist, enabling detailed analysis of their interaction in both wind speed and direction. For each selected case, night-time radiosoundings provide information on low-level jet height, inversion depth, and atmospheric stability, while high-resolution WRF simulations are analysed in detail to study the occurrence, phase, jet structure, and along-valley heterogeneity. 

The combined observational–modeling approach highlights the ability of NVWs to persist even under non-negligible synoptic forcing and provides insight into their vertical structure and spatial variability in complex terrain.