Spatio-temporal measurements of turbulence in the ABL with a fleet of small uncrewed aircraft systems

Exchange and transport processes in the atmospheric boundary layer (ABL) are driven by turbulence on a wide range of scales. Atmospheric turbulence is also a critical parameter for the determination of wind loads on structures such as wind turbines. Measuring turbulence with single point measurements requires assumptions about the stationarity, isotropy and homogeneity of the flow. The same applies if turbulence is retrieved from remote sensing instruments such as lidars. It is however well known that realistic turbulence does appear in more complex three-dimensional structures. This is the case in the ABL in stable and convective regimes, but even more so in complex terrain flow or in disturbed flow such as in the wake of wind turbines.
Without the infrastructure of meteorological masts and above applicable heights for such masts, in-situ point measurements of the three-dimensional wind vector are hardly available. 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. 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. The results are verified by measurements in the wind tunnel of the University Oldenburg with an active turbulence grid under laboratory conditions in 2022. First measurements in the near wake of a wind turbine were also conducted in 2022. 
Additionally to the wind vector estimation, pressure, temperature and humidity sensors are carried by each UAS. Most recently, a fine-wire resistance thermometer was added to the setup, which measuring temperature much faster than with standard sensors. Fast response measurements of temperature and vertical wind allow the calculation of sensible heat flux additionally to the Reynolds stresses.
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 2025.