Multirotor UAV observations of wind, thermodynamics, and shortwave radiation in proximity to eddy covariance towers

Accurate quantification of the Surface Energy Balance (SEB) in complex terrains remains an open challenge due to high spatial heterogeneity and scale mismatches between surface fluxes and atmospheric observations. Eddy Covariance Towers (ECT) provide continuous flux monitoring, but their spatial representativeness is limited in environments where atmospheric conditions vary over short distances, particularly in Alpine regions. Uncrewed Aircraft Vehicles (UAV) represent a complementary observation strategy by enabling distributed measurements of the near-surface variables that affect turbulent and radiative exchanges in sub-mesoscale ranges.

This work reports on the development, validation, and field deployment of a UAV-based measurement platform and data aggregation workflow for the spatial acquisition of wind, air temperature, relative humidity, and shortwave radiation in proximity to ECTs, to estimate the role of local circulations and advection on the SEB closure.

At first, computational fluid dynamics (CFD) simulations using the k–ε turbulence model were performed to assess propeller-induced aerodynamic distortions and to guide sensor onboarding on a multirotor UAV. Then, wind-tunnel experiments were executed at the WindShape facility (Switzerland), using a dedicated UAV testing setup comprising a 6-degree-of-freedom robotic arm for controlling UAV attitude and orientation. Lastly, a field campaign including repeated flights and hovering stops at predefined locations was carried out in Mezzolombardo (Italy) within the framework of the TEAMx international research programme.

By comparing ECT and UAV data, some general remarks can be made. In the case of temperature (T) and relative humidity (RH) measurements, sensors with fast response time are crucial for exploiting at best the limited UAV flight endurance. Our field tests highlighted T and RH maximum differences between the ECT HMP155A Vaisala sensor and UAV Galltech PM15P sensor measurements of 1.0 °C and 7%, respectively in a range of 100 m from the tower. However, this data is very fragmented due to the long acquisition time needed to get to steady-state values. In the case of wind measurements, no issues with sensor response time were noticed. Horizontal wind gusts up to 3 m/s were recorded by a TriSonica Mini LI-550 mounted on the UAV, while wind gusts up to 2 m/s were recorded by the Gill HS-100 anemometer of the ECT. Our preliminary results aim at demonstrating that multirotor UAV platforms have the capability of capturing information that ECT observations alone cannot resolve, provided that high-res. / high-freq. sensors are onboarded and conditioned according to the aforementioned procedure. This strategy (possibly empowered by UAV swarms) is expected to greatly contribute to the interpretation of flux footprints and assessment of horizontal heterogeneity in ongoing and future SEB closure studies.