High-Resolution Modelling of Down-Valley Flows in the Pyrenees

Thermally driven down-valley flows play a crucial role in the local meteorology of mountainous regions, governing the transport of mass, energy, and pollutants. Accurately representing these phenomena requires resolving the interactions between the atmospheric flow, large-scale winds, thermal forcing, and steep underlying orography. 

This study presents an in-depth investigation of down-valley flow dynamics in the Aure valley (French Pyrenees) using high-resolution simulations (200 m horizontal grid spacing) with the Weather Research and Forecasting (WRF) model. The analysis focuses on the physical processes and multiscale interactions occurring within the valley atmosphere.

To explicitly resolve the physical processes governing the generation and evolution of valley flows, an offline one-way nesting technique is implemented to bridge the gap between large-scale atmospheric forcing and sub-kilometer scales. While the accurate simulation of turbulence in the atmospheric boundary layer at these scales remains a complex issue, it offers significant advantages under stable conditions and over heterogeneous surfaces, as is the case of the valley considered here, where resolving small-scale surface features is desirable. This framework explicitly represents the development of the nocturnal surface inversion and topographically channeled flows, avoiding using the traditional planetary boundary layer parameterizations.

The numerical setup is evaluated against a comprehensive observational dataset from a field campaign conducted in summer 2023, including surface meteorological stations distributed across key valley locations and in situ radio soundings launched at different stages of nocturnal flow development. These vertical profiles are used to assess  the model’s ability to reproduce the depth, thermal stratification, and kinematic structure of the nocturnal down-valley flow.