On the descent of the Alpine south foehn

Abstract. When foehn winds surmount the Alps from the south, they often abruptly and vigorously descend into the leeside valleys on the Alpine north side. Scientists have long been intrigued by the underlying cause of this pronounced descent. While mountain gravity waves provide a modern theoretical foundation to explain the phenomenon, the descent of the Alpine south foehn has, so far, not been studied quantitatively and for a series of real-case events. To fill this research gap, the present study employs kilometer-scale numerical simulations, combined with online trajectories calculated during model integration, to investigate the descent process with unprecedented detail. Adopting the Lagrangian perspective, the locations of descent are explicitly identified and the key characteristics are determined, thereby encompassing foehn regions spanning from the Western to the Eastern Alps.

In the first part of the study, we find the descent of foehn air parcels to be primarily confined to distinct hotspots in the immediate lee of local mountain peaks and chains, underlining the fundamental role of local topography in providing a natural anchor for the descent during south foehn. Consequently, the small-scale elevation differences of the underlying terrain largely determine the magnitude of the descent. Combined with the fact that the descent is mostly dry-adiabatic, these results suggest that the descending motion occurs along downward-sloping isentropes generated by gravity waves.

The second part of the study aims to elucidate the different factors affecting the descent on a local scale. To this end, a particularly prominent hotspot situated along the Rätikon, a regional mountain range adjacent to the Rhine Valley, is examined in two detailed case studies. During periods characterized by intensified descent, local peaks along the Rätikon excite gravity waves, consequently leading to the descent of air parcels into the northern tributaries of the Rätikon and into the Rhine Valley. The two case studies reveal that different wave regimes, including vertically propagating waves, breaking waves, and horizontally propagating lee waves, are associated with the descent, indicating the lack of a preferential wave regime that is most conducive for descent along the Rätikon. In addition to gravity waves, other effects likewise influence the descent activity. First of all, a topographic concavity deflects the near-surface flow and thus promotes strong descent of air parcels towards the floor of the Rhine Valley. Secondly, nocturnal cooling can inhibit the formation of pronounced gravity waves and thus impede the descent of foehn air parcels into the valley atmosphere.

In summary, this study approaches a long-standing topic in foehn research from a new angle. Using online trajectories, the descent of foehn is explicitly identified and quantified, encompassing multiple case studies and a wide range of different foehn regions. The findings highlight the benefits offered by the Lagrangian perspective, which not only complements but also substantially extends the previously predominant Eulerian perspective on the descent of foehn.