Classification of Alpine south foehn based on 5 years of kilometre-scale analysis data

Abstract

It has long been recognized that a rich variety of Alpine south-foehn flavours exist that are related to varying flow conditions above crest level, the presence and intensity of orographic precipitation on the Alpine south side, and the Po Valley stratification. This study presents a systematic 5-year climatology of different foehn types. The classification relies on 2329 foehn hours, which are diagnosed using a station-based foehn index for Altdorf in the Swiss Reuss Valley. Operational analyses at 1 km horizontal resolution are employed to classify foehn hours with a decision tree that is based on foehn forecasting experience. Mean wind direction and speed in a circle with 100 km radius centred around Altdorf are considered to differentiate between three main foehn types (deep foehn, shallow foehn, gegenstrom foehn). In addition, upstream precipitation and its extent beyond the Alpine crest are used to distinguish three deep-foehn subtypes (dry foehn, moist foehn, dimmer foehn). The main foehn types differ distinctively in the synoptic conditions over the Alps. During deep foehn, pronounced southwesterlies ahead of an upper-level trough induce upstream orographic precipitation. Shallow foehn, in turn, is associated with cross-Alpine temperature differences that provoke a gap flow. The gegenstrom-foehn type is also restricted to major gaps, but a strong westerly flow prevails above crest level. The deep-foehn subtypes primarily differ in terms of the upper-level trough. While a weaker trough and the influence of an upper-level ridge over the Mediterranean inhibit precipitation (dry foehn), a deeper trough which is closer to the Alps induces stronger crest-level winds and intense precipitation on the Alpine south side (dimmer foehn). The different foehn types are found to strongly affect the local characteristics at Altdorf, which are investigated using station measurements. Backward trajectories from Altdorf are calculated for each of the foehn hours and used to define three clusters of air parcels depending upon their upstream thermodynamic evolution. Trajectories in cluster 1 are diabatically heated and transported within a low-level easterly barrier jet in the Po Valley prior to their ascent to crest level. They constitute the main precipitating airstream and, hence, are of key importance for moist foehn and dimmer foehn. Cluster-2 and cluster-3 trajectories are subject to weak diabatic heating or even diabatic cooling. They originate from southerly to southwesterly regions and from either slightly below or above crest level. Accordingly, these air parcels are associated with little to no precipitation, and as such, they take a key role for dry foehn, shallow foehn and gegenstrom foehn. Furthermore, these three foehn types feature a pronounced stable layer over the Po Valley, which, to some extent, inhibits air parcels to ascend from lower levels. In summary, the study introduces a systematic classification of south foehn using state-of-the-art data sets. It concludes by setting the new classification into a historic context and revisiting the rich body of literature with respect to different Alpine south-foehn types. In particular, analogies to and discrepancies with the existing conceptual models of “Swiss foehn” and “Austrian foehn” are discussed.