Atmospheric boundary layer structure at a small Alpine valley head detected with a network of UAS and ground-based sensors

In mountainous areas, the transport and exchange of mass, energy, and momentum in the atmospheric boundary layer (ABL) happens not only in the vertical, but also in the horizontal, and on multiple scales. The associated processes will be investigated within the observational campaign (TOC) of the TEAMx program at various locations. In a pre-campaign in 2022 (TEAMx-PC22), several sites, instrumentation, and measurement strategies were tested.

A site called Nafingalm was identified as a potential location for the investigation of boundary layer processes in a small Alpine valley. The site is located at the head of a tributary valley to the Inn Valley in Tyrol, Austria.

Measuring all the relevant scales within the ABL in the valley requires distributed sensors at the ground, but also aloft, ideally up to the boundary layer height. For this purpose, simultaneous, distributed measurements were conducted with quadrotor UAS from the SWUF-3D fleet during the TEAMx-PC22. Different configurations and flight strategies were tested between 20th and 28th June 2022 with up to three UAS being operated at the same time. The main flight strategies were simultaneous vertical profiles along the valley up to 120 m above ground, and horizontal profiles across the valley at up to 40 m above ground. Additionally, ground-based instrumentation was deployed during a three-month period to get a better understanding and statistics of typical conditions in the valley.

A case study was done for 23rd June 2022, analyzing the atmospheric processes that occurred during different periods of the day. It could be concluded that the UAS measurements were an important addition to and extension of the ground-based measurements. They illustrated 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.