Penetration and Interruption of Alpine Foehn (PIANO): Evaluation of Processes with Large-Eddy Simulations

During previous field campaigns, such as the Mesoscale Alpine Programme, a deep understanding of the well-established phase of foehn winds and associated mesoscale processes such as gravity-wave dynamics has been achieved. However, the initial and final stages of foehn have received far less scientific attention. Until now, no systematic study exists of the smaller-scale processes that control the onset and decay of foehn at the valley bottom. The research project ”Penetration and Interruption of Alpine Foehn” (PIANO) aims at increasing the understanding of these governing processes based on high-resolution numerical simulations as well as measurements conducted during a field experiment in the Inn Valley near the city of Innsbruck, Austria, in fall 2017. Especially the interaction between the downslope windstorm and the cold-air pool prior to foehn breakthrough is assessed. This conference contribution focuses on numerical simulations whereas highlights of the PIANO field campaign are presented in separate contributions.

The Weather Research and Forecasting (WRF) model is used to conduct mesoscale numerical simulations as well as large-eddy simulations (LES) of foehn during the PIANO Intensive Observation Period (IOP) 2 between 4 and 5 November 2017. The simulations are used to investigate the flow structure in complex terrain around the city of Innsbruck during foehn/cold-pool interaction. More specifically, the budgets of heat and turbulence kinetic energy are evaluated for the valley atmosphere to deduce the relative importance of different processes of cold-pool erosion. Previous studies showed that such processes are, for example, bottom-up heating by the surface sensible heat flux, top-down heating by shear-induced turbulent mixing at the foehn/cold-pool interface, or dynamical displacement of the cold pool. First results for IOP2 show that these processes occur all together but with different strengths at different locations in and around the city of Innsbruck. This complexity poses a challenge for mesoscale NWP models to correctly predict the timing of foehn breakthrough and interruption.