Thermodynamics and airstreams of a south foehn event in different Alpine valleys

Abstract

Foehn flows are associated with a characteristic warming of the respective air in lee-side valleys. Recent studies about the physical mechanisms question the traditional thermodynamic foehn theory, pointing out the potential role of adiabatic descent and other diabatic processes, besides upstream latent heating in clouds, for the warming. This study applies a kilometre-scale simulation together with online trajectories and a Lagrangian heat budget to investigate the foehn air warming for an intense, long-lasting Alpine south foehn period in November 2016. Thereby, the focus lies on the attribution of the warming to adiabatic descent and the different diabatic processes in six foehn valleys, as well as the linkage of the warming processes to different airstreams. Overall, adiabatic warming emerges as the most important process for 57% of all air parcels arriving in the foehn valleys. However, upstream latent heating in clouds is the dominant mechanism for a considerable amount of air parcels (35%). The heat budget analysis reveals a clear transition along the Alps from west to east, whereat diabatic warming constitutes the driving mechanism for western valleys during the central phase of the event, and adiabatic warming dominates the foehn air warming in eastern valleys. The foehn trajectories can be separated into different air-stream categories according to their diabatic temperature change. Air parcels experiencing strong diabatic heating are transported in an easterly barrier jet in the Po Valley before crossing the Alpine crest. Air parcels experiencing diabatic cooling, in contrast, are advected quasi-horizontally from the south towards the Alpine crest prior to their descent. The relative strength of the different airstreams therefore determines the dominating warming process. These findings strongly point to the combined importance of adiabatic and diabatic warming mechanisms, which co-occur with varying relative contributions, depending on the valley and time period of the event.