Towards spatio-temporal measurements in the mountain 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. Their adequate parameterization in numerical weather prediction (NWP) models is essential for a high predictive skill of forecasts. In heterogenous and complex terrain, the common simplification of turbulence to 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. A 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. In field campaigns with more than 1000 single flights at the Meteorological Observatory Lindenberg - Richard Aßmann-Observatory (MOL-RAO), the system was validated in 2020 and 2021 to provide reliable measurements of the horizontal wind vector. 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 atmospheric stratification. Additionally to the wind vector estimation that is based only on avionic data from the autopilot, pressure, temperature and humidity sensors are carried by each UAS.
In future, within the project ESTABLIS-UAS, the fleet of UAS shall be expanded and capabilities for flights beyond visual line of sight and throughout the whole ABL shall be developed. The project includes a three-fold approach to validate single UAS measurements, fleet observations and methods to derive spatial averages and fluxes. Wind tunnel tests, field experiments and virtual measurements in numerical simulations will be performed to gain confidence in the achievable accuracy in a wide range of conditions. Also, measurement strategies are to be developed that allow the derivation of meaningful fluxes in the mountain boundary layer (MoBL). 
The UAS fleet is planned to be deployed in two campaigns in the framework of the TEAMx research programme. The ESTABLIS-UAS measurements will fill observational gaps in the sub-mesoscale. The analysis of the UAS fleet data in synthesis with continuous ground observations and remote sensing will provide unprecedented new insights into the complex MoBL flow. The results will foster the development of new and better parameterization of the ABL in complex terrain.