Swiss and Austrian Foehn revisited: A Lagrangian-based analysis

Abstract

Two different South Foehn types have been described in the literature: the Swiss Foehn is characterized by significant ascent on the southern side of the Alps, hence fulfilling a requirement of the thermodynamic Foehn theory associated with latent heating. On contrast, the Austrian Foehn is characterized by near-horizontal flow to the south of the Alps, followed by dry-adiabatic descent into the northern Foehn valleys. In this study, we make use of three years (2000-2002) of NWP reanalysis data, based on the COSMO model, and corresponding Foehn observations at a Swiss (Altdorf in the Reuss Valley) and Austrian (Ellbogen in the Wipp Valley) measurement site to address the applicability of this Foehn type classification. First, the methods are introduced in a case study of a strong Foehn case on 2-4 April 2000. The more traditional Eulerian analysis is complemented by trajectory calculations. Forward trajectories started in the Po Valley and backward trajectories started at the two Foehn stations reveal a complex flow situation. For instance, air parcels arriving in Altdorf can be trapped in the easterly, low-level Po Valley jet before ascending and passing over the Alpine crest, thus originating from further east than air parcels arriving in Ellbogen and having experienced less vertical ascent south of the Alps. This highlights the potential of Lagrangian-based flow analysis and concurrently points to the limitations of a pure Eulerian perspective. The main part of the study considers a climatology of the 3-year backward trajectories started at Altdorf and Ellbogen. Some key findings are: (i) a larger fraction of trajectories arriving in Altdorf experience substantial lifting on the Alpine south side compared to Ellbogen;(ii) both Foehn types can be observed at both stations, i.e., the type naming cannot be taken as an exclusive regional classification; (iii) precipitation traced back is more predominant for Altdorf trajectories than for Ellbogen ones, indicating that the latent heating contributes more to Foehn warming in Altdorf than in Ellbogen. Finally, from a forecasting perspective it is of interest whether the Foehn type can be deduced from a simple measurement alone. To this aim, Milan pseudo-soundings in the Po Valley, essentially south of Altdorf, are considered. Composite soundings are compared for Foehn cases in Altdorf with substantial lifting to those with weak lifting, corresponding to a non-blocked and blocked flow in the Po Valley. The two classes clearly differ in their composite profile;however, the spread prohibits an immediate classification on this sounding alone.