Foehn–cold pool interactions in the Inn Valley during PIANO IOP2

Foehn in Alpine valleys is characterized by a complex, transient wind field and strong turbulence. This has implications for air pollution in valleys and poses a hazard to aviation. For example, in the Inn Valley aircraft have to approach Innsbruck Airport from opposite directions prior to and after foehn breakthrough to ensure safety. Therefore an accurate forecast of foehn breakthrough and interruption in the valleys is crucial for aviation safety and air quality prediction. However, in order to improve the forecast, processes which control these transient phases of foehn onset and decay have to be better understood.

These processes are highly three-dimensionals and involve a broad spectrum of scales. E.g., on the mesoscale, gravity waves have the potential to displace the underlying cold-air-pool (CAP) and to pave the way for the foehn flow to penetrate into the valley. This process can be amplified by shear flow instability which occurs at the foehn-CAP interface and leads to turbulent erosion of the CAP from the top. A bottom-up destruction of the CAP can be accomplished by a positive sensible heat flux during daytime. To assess the relative contribution of these different processes to the total CAP erosion, a measurement campaign took place in autumn and early winter 2017 in the Inn Valley, Austria. The campaign is part of the research project “Penetration and Interruption of Alpine Foehn (PIANO)” and included a dense network of instruments: temperature and humidity loggers, automatic weather stations, radio soundings, eddy-covariance stations and four Doppler wind lidars. From the latter, spatio-temporal information about the three-dimensional wind field was retrieved (see Figure).

In this work, we present a case study of a south foehn observed during the second Intensive Observation Period of the 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. A few hours after the second breakthrough 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.

 

Mean daily precipitation over New Zealand

Figure: Two-dimensional wind vectors retrieved from coordinated dual Doppler measumements along a south-north orientated vertical plane above the city of Innsbruck. The wind field illustrates the formation of shear flow instabilities in the transition zone of the cold-air pool and the upper-level foehn flow

Haid, M., Gohm, A., Umek, L., Ward, H.C., Muschinski, T., Lehner, L. and Rotach, M.W.: Foehn‐cold pool interactions in the Inn Valley during PIANO IOP2. Quarterly Journal of the Royal Meteorological Society. https://doi.org/10.1002/qj.3735, 2020.


Previous editions of Science Flash

Johannes Horak: Assessing the added value of the Intermediate Complexity Atmospheric Research Model (ICAR) for precipitation in complex topography (2019)

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