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In dieser Arbeit wird die komplexe Wechselwirkung zwischen orographisch induzierten Fallwinden, in den Alpen als Föhn bekannt, und Kaltluftseen (Cold Air Pools, CAPs) untersucht. Föhn zeichnet sich durch relativ warme und trockene Luftmassen, hohe Windgeschwindigkeiten und oft starke Turbulenz aus. Im Gegensatz dazu wird ein CAP als eine bodennahe Luftschicht mit relativ kalter Lufttemperatur und hoher statischer Stabilität charakterisiert, welche sich in einer topografischen Senke oder einem Tal befindet. Diese potentiell kältere Luftschicht kann verhindern, dass die potentiell wärmeren Föhnwinde den Talboden erreichen. CAPs entstehen oft durch nächtliche Abkühlung vor Aufkommen des Föhns. Daher kann in den frühen Stadien eines Föhnereignisses häufig eine komplexe Wechselwirkung zwischen der Föhnströmung und dem darunter liegenden CAP beobachtet werden. Allerdings sind CAPs nicht nur im Früh-, sondern auch im Spätstadium eines Föhnereignisses relevant, da ihre nächtliche Regeneration zu vorübergehenden Föhnpausen am Talboden führen kann. Die Übrgangszone zwischen CAP und Föhn weist häufig starke horizontale und vertikale Windscherung, starke vertikale Gradienten der statischen Stabilität und starke Turbulenz auf. Daher stellt die Erfassung turbulenter Prozesse, wie z.b. Kelvin-Helmholtz Instabilitäten welche an dieser Föhn-CAP-Grenzfläche wirken, eine Herausforderung nicht nur für numerische Wettermodelle sondern auch für Messinstrumente dar. Neben der turbulenten Erosion des CAP durch die Föhnströmung von oben nach unten kann ein CAP auch durch Erwärmung von unten, verursacht durch den sensiblen Wärmestrom, sowie durch horizontale Advektion durch die Föhnströmung entfernt werden. Um den Zeitpunkt des Einsetzens und Abklingens des Föhns besser vorhersagen zu können, müssen die damit verbundenen atmosphärischen Prozesse quantifiziert und besser verstanden werden. Um ein besseres Verständnis dieser Prozesse zu erreichen, wurde die Feldkampagne des Forschungsprojekts, In this thesis, the complex interaction between severe downslope windstorms, known as foehn in the Alps, and cold-air pools (CAPs) is investigated. Foehn winds are characterized by relatively warm and dry air masses, high wind speeds and often severe turbulence. In contrast, a CAP is a surface-based atmospheric layer of relatively cold air characterized by high static stability, located in a topographic depression like a valley. This potentially colder air can prevent the potentially warmer foehn winds from reaching the valley floor. Such CAPs are often formed by nocturnal cooling prior to foehn. Therefore, in the early stages of a foehn event, there is often a complex interaction between the foehn flow aloft and the CAP below. However, CAPs are relevant not only in the early stages but also in the later stages of a foehn event, as their regeneration at night can lead to temporary foehn breaks. The foehn-CAP interface often exhibits large horizontal and vertical wind shear, strong vertical gradients in static stability and turbulent motions. Therefore, capturing turbulent processes such as shear-flow instability acting at this foehn-CAP interface represents a challenge not only for numerical models, but also for observing systems. Beside turbulent erosion of the CAP by the foehn flow from the top-down, a CAP can also be dissipated by bottom-up heating through the surface sensible heat flux as well as horizontally displaced by the foehn flow. In order to improve forecasting of the timing of foehn onset and decay, the associated atmospheric processes need to be quantified and better understood. To achieve a better understanding of such processes, the field campaign of the research project Penetration and Interruption of Alpine Foehn (PIANO) was conducted in the complex terrain around Innsbruck, Austria. This area is often influenced by strong south foehn. During such events, Innsbruck is located downstream of the Wipp Valley, one of the major Alpine foehn valleys. The, by Lukas Umek, MSc., Kumulative Dissertation aus drei Artikeln, Dissertation University of Innsbruck 2023

First, a list of 30 potential lake-effect precipitation events between 01 September 2008 and 31 December 2013 is compiled. These events were characterized by a difference between the LST und the mean 2-m air temperature in Bregenz of 2 to 10 K. Maximum observed precipitation accumulated over 36 h varied between about 4 and 126 mm. More than 70 % of the events occurred between July and December. One event isThis is one of the first case studies of a snowstorm at Lake Constance, located between Austria, Germany and Switzerland, which assesses the influence of the lake and the orography on the generation of heavy precipitation. The analysis is based on surface and radar observations as well as numerical simulations with the Weather Research and Forecasting (WRF) Model. Sensitivity experiments are conducted with modified surrounding orography, land-use and lake surface temperature (LST) as well as various different microphysics parameterizations to determine the role of individual physical processes. First, a list of 30 potential lake-effect precipitation events between 01 September 2008 and 31 December 2013 is compiled. These events were characterized by a difference between the LST und the mean 2-m air temperature in Bregenz of 2 to 10 K. Maximum observed precipitation accumulated over 36 h varied between about 4 and 126 mm. More than 70 % of the events occurred between July and December. One event is analyzed in detail in this work. The selected event occurred on 08 February 2013 during postfrontal conditions with northwesterly low-level flow along the long axis of the lake. It is characterized by a rather stationary and banded radar reflectivity pattern. The associated snowband reached a length of up to 30 km and affected the downstream shore and the adjacent mountainous region. The maximum of approximately 36 mm snow water equivalent accumulated within about 5 hours occurred in Bregenz at the eastern shore of the lake close to sloping terrain. The analysis of surfa, by Lukas Umek, Innsbruck, Univ., Master-Arb., 2016