Added value of off-the-shelf UAS for exploring Alpine valley flows

Scanning Doppler wind lidars have become an important tool for investigating the kinematic structure of the mountain boundary layer and associated local flows. However, currently, no appropriate remote sensing technique exist that can capture the thermodynamic structure (temperature and humidity) in the lowest hectometres above ground at a sufficient spatiotemporal resolution. Hence, uncrewed aircraft systems (UAS) have become a prominent tool to fill this gap.

In this work, we report how a single off-the-shelf UAS (DJI Mini 2) equipped with a temperature and humidity logger (iMet-XQ2) can provide added value for the interpretation of Doppler wind lidar observations of complex winds in a narrow Alpine valley. The study site is located at Nafingalm, a mountain pasture located at the end of the Weer Valley in Tyrol, Austria. This location will be one of the target areas of the TEAMx Observational Campaign (TOC) in summer 2025. We present data from a short campaign conducted on 01 and 02 September 2023 to test the feasibility of combined UAS and Doppler lidar measurements at this remote site. The UAS performed vertical profiles over more than 24 hours of the lowest 120 m above ground at a 30-minute interval during daytime and an hourly interval during nighttime to capture the whole boundary layer evolution.

We will show the characteristics of daytime upvalley and nighttime downvalley winds as captured by the Doppler wind lidar and the corresponding temperature structure depicted by the UAS observations. In this context, the UAS measurements were crucial for correctly interpreting the transient warming phases during early evening as turbulent mixing events resulting from the interaction of a cross-mountain airflow with the stable boundary layer in the valley. The observations indicate that the early evening transition phase is characterized by high complexity and presents an interesting phase for studying turbulent processes in more detail within the framework of the TOC.