Using the SWUF-3D UAS fleet to determine heat flux characteristics in an Alpine valley

Single multirotor uncrewed aerial systems (UASs) are rapidly moving towards operational profiling in the atmospheric boundary layer (ABL) because of their low cost and ease of operation. At the same time, these features allow researchers to deploy such systems for advanced campaign-based sampling strategies. Using a fleet of UASs allows the flexibility to sample the spatiotemporal structure of turbulence in the ABL. Such observations are particularly useful in complex terrain where it is difficult to sample with classical approaches. During the TEAMx campaign, the DLR SWUF-3D fleet of UASs was operated in a remote Alpine valley. Each SWUF-3D UAS is outfitted with a rapid response temperature sensor and can determine the 3D wind field at 5 Hz; these measurements are calibrated in-field against a sonic anemometer. The 3D box-pattern configuration of a UAS fleet hovers up to 18 min at fixed-position across the valley and allows spatial gradients to be calculated. In July 2025, 88 box-pattern fleet flights were completed across a range of weather conditions. Valley heating mechanisms are unique due to contributions in all three dimensions but rarely have the observations to characterize the volume effects. The flexibility of UAS deployment provides the opportunity to analyze the heating budget terms with in-situ observations. In valleys, the horizontal heat flux contribution is no longer negligible and varies with proximity to valley walls. After verifying the UAS fleet observations of heat flux against a sonic anemometer, the range of uncertainty is demonstrated. Then the heterogeneity of heat flux observations will be related to atmospheric stability using vertical UAS profiles throughout the diurnal cycle.