Abstract
A case‐study is presented of a south foehn emanating from the Wipp Valley, Austria, which encountered a cold‐air pool (CAP) in the Inn Valley near the city of Innsbruck. The analysis is based on data collected during the second Intensive Observation Period of the Penetration and Interruption of Alpine Foehn (PIANO) field experiment. Foehn was initiated on 3 November 2017 by an eastward moving trough and terminated in the afternoon of 5 November 2017 by a cold front passage. On two occasions, reversed foehn flow deflected at the mountain ridge north of Innsbruck penetrated to the bottom of the Inn Valley. The first breakthrough occurred in the afternoon of 4 November 2017. It was transient and locally limited to the northwest of the city. The second (final) breakthrough occurred in the morning of 5 November 2017 and was recorded by all surface stations in the vicinity of Innsbruck. It started with a foehn air intrusion to the northeast of Innsbruck and continued with the westward propagation of the foehn–CAP boundary along the valley. Subsequently observed northerly winds above the city were caused by an atmospheric rotor. A few hours later and prior to the cold front passage, the CAP pushed back and lifted the foehn air from the ground. During both nights, shear flow instabilities formed at the foehn–CAP interface, which resulted in turbulent heating of the CAP and cooling of the foehn. However, this turbulent heating/cooling was partly compensated by other mechanisms. Especially in the presence of strong spatial CAP heterogeneity during the second night, heating in the CAP was most likely overcompensated by negative horizontal temperature advection.
Highlights
Foehn in Alpine valleys is characterized by a complex, transient wind field and strong turbulence
The goals of this work are to analyse the evolution leading to the final foehn breakthrough and interruption and to identify the dominant processes of foehn–cold-air pool (CAP) interaction relevant for the CAP heat budget
A conceptual model is presented for the foehn–CAP interaction during the two IOP nights, the foehn breakthrough and breakdown
Summary
Foehn in Alpine valleys is characterized by a complex, transient wind field and strong turbulence. This has implications for air pollution in valleys (e.g., Seibert et al, 2000; Furger et al, 2005; Gohm et al, 2009) and poses a hazard to aviation (e.g., Gohm et al, 2008; Chan and Hon, 2016). Often prior to foehn breakthrough, the atmosphere in Alpine valleys is characterized by a stably stratified cold-air pool (CAP) at the valley bottom and potentially warmer foehn flow aloft. Gubser and Richner (2001) mention three potential mechanisms of CAP erosion during foehn: (1) bottom-up heating of the CAP, (2) top-down heating by turbulent mixing, and (3) CAP displacement These processes are not exclusive to foehn in the Alps. They play a role in cold-pool studies in other regions in the absence of an upper-level foehn flow (e.g., Price et al, 2011; Lareau et al, 2013)
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