Atmospheric boundary layer structure at the head of a small Alpine tributary valley detected with uncrewed aerial systems (UAS)

Kurzfassung

Wind systems in the atmospheric boundary layer within valleys contribute to the transport and exchange of mass, energy, and momentum in mountainous areas. The research program "Multiscale transport and exchange processes in the atmosphere over mountains" (TEAMx) aims to investigate these processes on different scales through observations and numerical simulations in the TEAMx Observational Campaign(TOC) in 2024/2025. Measurements will be conducted at multiple sub-target areas across the Alps one of which being Nafingalm, a head of a tributary valley in Tyrol, Austria. In the framework of a pre¬ campaign in 2022(TEAMx-PC22), ground-based and airborne measurements were done at Nafingalm to explore the local mountain boundary layer and to test instrumentation for the TOC. Ground-based instruments were deployed between 15 June and 12 September 2022. Flights with a fleet of uncrewed aerial systems (UAS) were performed between 20 and 28 June 2022. The UAS measured temperature and humidity by an external sensor. Wind speed and direction were determined from the attitude data of the drones. Flight patterns performed were simultaneous point measurements, vertical and horizontal profiles, and combinations of these patterns. A case study was done on 23 June 2022 when most UAS flights were available, analyzing the atmospheric processes that occurred during different periods of the day. The UAS measurements complemented and extended the ground-based measurements, illustrating the occurrence of thermally-driven winds, foehn winds, and the formation of a stable boundary layer at the valley ground. Nafingalm proved to be a suitable location to observe local and mesoscale phenomena and their interaction. For the TOC, the UAS measurements should be complemented with remote sensing and further ground-based in-situ instrumentation in order to measure wind as well as temperature and humidity distribution along the valley axis and at heights up to crest level. A larger drone fleet would allow to sample areas on the scale of a cross-valley circulation. Doppler wind lidars could provide insight into along-valley and cross-valley winds at levels higher than the maximum UAS flight level. Ground-based instrumentation at the slopes could compensate for the inability of the lidar and UAS to measure in the range of a meter from the valley side wall.