Towards spatio-temporal turbulence measurements in the atmospheric boundary layer with a fleet of UAS

Exchange and transport processes in the atmospheric boundary layer (ABL) are driven by turbulence on a wide range of scales. In heterogenous and complex terrain, the common simplification of turbulence to one-dimensional statistical models does not necessarily hold. Coherent structures such as convective cells, secondary circulations, gusts, slope and valley flows can be summarized to sub-mesoscale structures which are not well represented in models. Part of the reason for the lack of understanding of these flow features is the challenge to adequately sample their three-dimensional, spatio-temporal structure and their contribution to the energy budget of the ABL.
We present a system to achieve simultaneous spatial measurements with a fleet of multirotor unmanned aircraft systems (UAS). The major benefit of this approach is, that true simultaneous measurements can be obtained without the need of expensive infrastructure such as masts or lidar instruments. The SWUF-3D (Simultaneous Wind measurement with Unmanned Flight systems in 3D)  fleet was first deployed at the Meteorological Observatory Lindenberg - Richard Aßmann-Observatory (MOL-RAO) of DWD in association to the FESSTVaL campaign. With more than 1000 single flights, the system was validated against sonic anemometers at the 99-m mast in 2020 and 2021 and was able to provide reliable measurements of the wind vector with a RMSE below 0.3 m/s. We showed that turbulent eddies can be resolved with a time resolution of up to 2~Hz, unless the overall TKE level is below the noise threshold of the UAS measurements, which can be the case in stable ABL conditions. We show that fluctuations of the vertical wind component can be captured with the system, if calibrated motor thrust data is used for the estimation. This allows to calculate TKE, momentum fluxes and friction velocity in neutral and convective atmospheric stratification. Additionally to the wind vector estimation, which is done with avionic data of the autopilot, pressure, temperature and humidity sensors are carried by each UAS.
The highlights of the two-week long campaign during FESSTVaL were spatial measurements of a gust front associated to a thunderstorm and a 4.5-hour long continuous vertical profiling of the atmosphere during a convective boundary layer morning transition period. Results of these measurements will be presented. 
Within the project ESTABLIS-UAS (Exposing Spatio-temporal structures of Turbulence in the ABL with In-Situe Measurements by UAS, funded by the European Union), the SWUF-3D fleet will be enhanced to allow operation in larger areas and in complex terrain. A fleet of up to 100 UAS shall be deployed during TEAMx (Multi-scale transport and exchange processes in the atmosphere over mountains – programme and experiment) in 2024.