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

Abstract

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.

A station‐based evaluation of near‐surface south foehn evolution in COSMO‐1

“…Continuous precipitation is observed on the southern side of the Alps during the whole foehn event. Based on the foehn classification method from Jansing et al (2022) (see Section 2.5), this foehn case is classified as a moist deep foehn with a few hours classified as Dimmer foehn (16 out of 108 hr).…”

“…Therefore, we combine a preselection of foehn cases/hours based on existing foehn indices with local foehn criteria to achieve a similar result. For the preselection, an enhanced offline version of the operational station‐based foehn index maintained by the Alpine Research Group Foehn Rhine Valley/Lake Constance (Arbeitsgemeinschaft Föhnforschung Rheintal–Bodensee, AGF) is used (Jansing et al, 2022). As for local foehn criteria, we utilize a simplified version of the AGF foehn definition.…”

“…To test the validity of the simplified AGF foehn criteria, we apply it to the five foehn cases. The resulting time indices determined from observations are compared with the enhanced offline version of the operational station‐based foehn index (hereafter called the “enhanced foehn index”: (Jansing et al, 2022). The enhanced foehn index is based on the operational foehn index available at MeteoSwiss with a time resolution of 10 min, where three foehn labels are defined: (0) no foehn, (1) foehn mixed air, and (2) foehn.…”

“…Following Jansing et al (2022), we classified all the foehn events that occurred within the five‐year period into five categories, including three deep foehn types: dry foehn, moist foehn, and Dimmer foehn, and other two foehn types: shallow foehn and Güller/Gegenstrom foehn. The major difference between deep foehn, shallow foehn, and Gegenstrom foehn is the upper‐level wind field over Altdorf (ALT).…”

“…Gerstgrasser (2017) summarized five south foehn types based on the synoptic to mesoscale conditions: (1) the classical foehn, which is driven by a low‐pressure system over the British Isles or the Bay of Biscay, which facilitates cold‐air blocking on the south side and warm‐air advection on the northern side of the Alps, causing a hydrostatic pressure gradient across the Alps that initiates the foehn flow; (2) the anticyclonic foehn, in which the low‐pressure system is further west, resulting in no precipitation and low humidity on both sides of the Alps (analogous to the Austrian foehn type); (3) the shallow foehn, which occurs with only weak support by the synoptic situation, driven by a deep cold pool on the south side of the Alps flowing through a mountain pass (the so‐called “waterfall theory”); (4) the Güller/Gegenstrom foehn as a special type of shallow foehn, in which the airflow above the mountain top is against the low‐level foehn flow direction (Güller, 1977); and (5) the Dimmer foehn, in which clouds and precipitation can exist on the north side of the mountain due to a strong cross‐mountain pressure gradient. Jansing et al (2022) summarized more foehn classification methods in detail.…”