Investigating Foehn Descent Hotspots in the Swiss Alps Using LES-Based Trajectory Analysis

Foehn winds are warm, dry downslope flows that occur on the leeward side of mountain ranges. In the Alps, the descent of foehn winds is often restricted to specific hotspots, where the interaction between complex terrain, mountain-induced gravity waves, and flow separation concentrates the descending air. These descent hotspots are associated with localized warming and drying, significantly affecting weather conditions, forecast uncertainty, and have implications for ecosystems and human activities in the region. Previous studies using the COSMO model—run at 1 km resolution—have visualized these hotspots and linked them to gravity wave dynamics. However, whether a 1 km resolution is sufficient to accurately resolve near-surface flow separation—a key factor in foehn dynamics and their predictability—remains an open question. 

To investigate this question, we conducted large-eddy simulations for a case study in the Rhine Valley (February 2017). Simulations were performed with the Icosahedral Nonhydrostatic (ICON) model using three nested domains with horizontal grid spacings of 520 m, 260 m, and 130 m, respectively. Turbulence at all three domains was represented using a 3D Smagorinsky closure. Additionally, we used offline Lagrangian trajectories to identify the descent pathways of foehn air parcels, enabling a detailed evaluation of how model resolution affects the spatial distribution of descent hotspots in the Alps. This study is the first to combine trajectory analysis with LES in the context of foehn research, allowing for a detailed visualization of foehn descent pathways. 

Preliminary results are consistent with previously observed hotspots of foehn descent in the Alps, confirming the Rhine Valley as a key pathway for flow descent. The results show that while some flow descents are indeed channelled along the predominantly north–south oriented valley, the main hotspots of foehn descent tend to accumulate along the east–west segments. This indicates that the most frequent descent pathways are controlled not by valley-parallel channelling, but by direct downslope acceleration on the north-facing slopes. The main features of the descent—particularly the typical origin height of trajectories between 500 m above ground level and the mean Alpine crest height, and the descent travelled distance—are preserved as grid spacing is refined from 500 m to 250 m. However, notable differences emerge at higher resolution, including a 30% increase in the number of descending trajectories in the valley and the development of faster and more intense descent events associated with stronger foehn jets. These findings indicate that while the essential foehn dynamics are captured at mesoscale resolutions, higher-resolution configurations are required to represent the finer-scale features and intensity of foehn descent in the Alps.