Turbulence-resolving Spatio-temporal measurements of ABL flow with a large fleet of multicopter UAS.

Small uncrewed aerial systems (UAS) are platforms which have been introduced into every-day life within the last decade due to their low cost, good availability and ease of use. They serve a large variety of applications, including aerial photography and cinematography, but also scientific purposes in earth observation. In atmospheric sciences, fixed-wing UAS have been used at first to collect in situ measurements especially in the atmospheric boundary layer (ABL). The way data was collected was based on common know-how from piloted research aircraft. Flow probes and fast-response sensors were installed to measure thermodynamic variables and derive turbulent fluxes. This study focuses on small multicopter UAS which are the most common type of UAS and usually referred to as `drones'. These systems are easier to operate due to their capability of vertical take-off and landing and advanced control systems. For the most part in atmospheric measurements, multicopter UAS are applied to collect vertical profiles of wind, temperature and humidity. For such profiling tasks, similar sensors as in radiosondes can be deployed and provide a good accuracy for temperature and humidity, but resolving turbulence is usually not the primary focus. However, if multiple systems can be placed at most flexible locations within the ABL to observe thermodynamic features at a high resolution, this enables a variety of new possibilities for research. We show that with the DLR SWUF-3D (simultaneous wind measurement with a UAS fleet in 3D) quadrotor fleet that consists of 35 UAS, turbulence eddies can be resolved with a frequency of up to 2 Hz by the individual drones. This applies for 3D wind measurements as well as for temperature measurements with a newly developed fine-wire platinum resistance thermometer (FWPRT). Within the limits towards the smallest scales we show that the data can be used to calculate fluxes of momentum and sensible heat with reasonable uncertainties in many atmospheric conditions. Additionally to field measurements, the UAS were calibrated and the results were verified in a wind tunnel setup. We show how data of the SWUF-3D fleet can be used to calculate spatial correlation and coherence in ABL flow. The system was also used to measure complex flow in an Alpine valley and in the near wake of a wind turbine.  An overview of the applications is given to show the potential of turbulence-resolving, spatio-temporal measurements with a large fleet of multicopter UAS.