Penetration and Interruption of Alpine Foehn (PIANO)

Downslope windstorms represent one of the greatest weather hazards in mountainous regions, apart from thunderstorms and heavy precipitation. Alpine foehn is a prominent representative of these types of windstorms. During the last decades, the scientific community has put ongoing effort into better understand and predict these severe winds. However, previous research has primarily focused on the well-developed stage rather then the complex initial and final stage of foehn. Hence, the mechanisms of foehn penetration into valleys and of foehn breakdown are still poorly understood. However, these transient stages have the strongest effect on aviation safety and air quality in the valley. Breakthrough and decay of foehn are controlled by various atmospheric processes. Some of the previous studies disagree on which of these processes are dominant. Furthermore, foehn poses a great challenge for numerical weather prediction models. Forecast errors are partly related to deficiencies in correctly capturing these processes. Therefore, the goal of this research project is to improve the understanding of atmospheric processes that govern the breakthrough and decay of foehn. Our methodology is based on a combination of sophisticated measurement techniques and computer simulations. A key element of the project is a field experiment that will be conducted in Innsbruck (Inn Valley, Austria) and its surroundings. The most important atmospheric observing systems are three combined laser-based instruments (so-called lidars) to remotely measure the turbulent flow field in the valley as well as a research aircraft to observe the airflow at flight level. To fill the spatial gaps of the measurement systems, the airflow will be simulated with a computer model (Weather Research and Forecasting Model). Observations and simulations will be used to quantify all relevant atmospheric processes. The computer model will not only be used for real-case studies but also as a virtual laboratory to evaluate the sensitivity of foehn onset and decay to various factors. The aim of the project is to compile a comprehensive picture of different prototypes of foehn development. The outcome of the project will be beneficial for improving the forecast skill of numerical weather prediction models over complex terrain. This, in turn, will improve severe turbulence and air pollution warnings.

Why does foehn descend into valleys? The Austrian meteorologist and mountaineer Heinrich von Ficker asked himself this question already 90 years ago at the beginning of his scientific essay on this stormy alpine wind. The PIANO research project was less about the question "Why?" but much more about the question How exactly?. Heinrich von Ficker already knew that foehn winds only establish at the valley floor when the cold air pool in the valley has been eroded. Until recently, however, there was still disagreement about the role of various erosion processes. In addition, the representation of these processes in numerical weather prediction models is incorrect and leads to erroneous forecasts of foehn breakthrough and breakdown. In the PIANO research project, these processes in the area around Innsbruck in the Inn Valley were examined and quantified in order to create the basis for future model improvements. For this purpose, detailed measurements were conducted in a field experiment and high-resolution flow simulations were carried out on high-performance computers. It was found that the turbulent erosion of the cold air pool from top to bottom due to mixing of foehn and cold air as a result of turbulent eddies is more important than assumed in the past. It was also shown that this process is not correctly captured in the forecast models. At the same time, it was found that the supply of cold air close to the ground from undisturbed regions of the cold pool by the so-called pre-foehn westerlies dampens the turbulent erosion. Thus, a temporary equilibrium can occur between cooling through horizontal transport and heating through vertical mixing. This equilibrium is easily disturbed if either one of the two components changes slightly or another component is added, such as warming due to solar radiation after sunrise. The various warming and cooling processes vary greatly in complex alpine terrain. In the study area, this variability can lead to foehn breakthrough in the east of the city of Innsbruck, while the cold air pool persists in the west. The resulting small-scale air mass differences are expressed in strong gradients in temperature, wind speed, turbulence intensity, and near-surface concentration of air pollutants. Foehn breakdown can occur not only by nocturnal cooling of the valley atmosphere or by the passage of a cold front, but also through the backflow of cold air pool remains immediately before the frontal passage. Several different patterns were thus identified for foehn breakthrough and breakdown. At the same time, it was shown that the details of the foehn evolution are highly case-dependent.