Foehn Dynamics - Lagrangian Analysis and Large-Eddy Simulation

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This project considers some of the key problems of Foehn research: the origin and warming mechanisms of the Foehn air, the descent of Foehn into the northern valleys and displacement of the cold air pool, the variability of the mechanisms for different Foehn flavors, and the accuracy of Foehn flows in NWP models. More specifically, the aims are: (i) The detailed Lagrangian analysis of Foehn air warming and of the scrambling of air masses will be extended to several (distinct flavors of) Foehn flows, hence assessing the impact of microphysical processes on warming and scrambling; additionally, the heat budget of the air parcels along the Foehn trajectories will include heating due to turbulence and radiation; (ii) The origin of the Foehn air in the valleys will be assessed by labeling and then transporting a passive tracer in the NWP model; in particular, the initial altitude of the air parcels on the Alpine south side will be used as a tracer. This kind of analysis will extend existing studies on the vertical evolution of Foehn air before it passes over the Alpine crest. As an additional benefit, labeling air masses above the Alpine crest and later determining their relative contribution in the Foehn valleys allows the degree of downward mixing to be quantified; (iii) Eulerian analysis of heat, moisture and momentum budgets and large-eddy simulations (LES) will be performed for selected Foehn valleys, resulting in a better understanding of the land-atmosphere coupling between Foehn air and the surface, in a clearer picture on how these processes influence the displacement (erosion) of the initial cold pools, and in a better understanding of the weaknesses and strengths of current and recently developed parameterizations of these processes in the NWP model used; (iv) The analyses planned for the selected case studies will, with a reduced degree of detail, be extended over a climatological time scale of (potentially) 10 years. To this aim, the analysis tools will be coupled to a novel approach in NWP simulations and analysis (Sinergia project crCLIM), where the NWP simulation and analysis tools run in parallel and communicate with each other by means of a data virtualization layer. The project will extensively use hindcast simulations of Foehn cases by the COSMO NWP model. The high- resolution trajectories will be calculated based on these hindcasts. Further, the LES simulations will address the complex flow evolution near the surface. Observational datasets will be used to evaluate the numerical simulations. The Eulerian budget analysis and Lagrangian analysis will complement each other and hence allow for a comprehensive analysis of the Foehn flow. The project will harvest recent methodological advances to shed new light on several long- standing problems in Foehn research and contribute to a better representation of Foehn in NWP models and hence lead the way for improved forecasts of a key high-impact weather phenomenon in Alpine regions.