Comparison of temperature structure parameter measurements by UAV and scintillometer during LITFASS-2009

The turbulent exchange of heat and water vapour is an essential land surface atmosphere interaction process in the local, regional and global energy and water cycles. Scintillometry can be considered as the only technique presently available for the quasi-operational experimental determination of area-averaged turbulent fluxes needed to validate the fluxes simulated by regional atmospheric models or derived from satellite images at a horizontal scale of a few kilometres. A field experiment (LITFASS-2009, Lindenberg-to-Falkenberg: Aircraft, Scintillometer and large-eddy Simulation) was organised in July 2009 at the Meteorological Observatory Lindenberg / Richard-Aßmann-Observatory of the German Meteorological Service (DWD) in order to study some fundamental issues related to the applicability of scintillometry. The turbulent structure parameter (CT2) over a heterogeneous agricultural landscape was measured by a small unmanned aircraft system (UAS) and by large-aperture scintillometers (LAS). The data was analysed regarding the implications for the interpretation of scintillometer measurements. The applied UAS is the meteorological mini aerial vehicle (M²AV) T200 'Carolo', which is a self-constructed model plane with two electric engines and a wingspan of two metres. The maximum take-off weight is 6 kg, including 1.5 kg of payload. With an endurance of approximately 50 minutes, the range accounts for about 60 km at a cruising speed of 22 m/s. The research UAV is controlled by an autopilot and therefore it operates automatically. This allows for measurement flights in the lower ABL over larger distances outside the range of sight and in remote areas. The meteorological instrumentation of the UAS consists of a five-hole probe for measuring the turbulent wind vector, two temperature sensors (fast and slow response), and a humidity sensor (humicap). Turbulence measurements are sampled at 100 Hz, resulting in a resolution (after anti-aliasing filters) of about 30 Hz (except for humidity data). In contrast to the LAS, the CT2 was calculated directly from in-situ measured air temperature time series. Finally measurements of both UAS and LAS were compared to identify any systematic discrepancies. In LITFASS-2009 special emphasis was put on the determination of the spatial and temporal variability of CT2 over moderately heterogeneous terrain. First results show in principle good agreement between CT2 measured by UAS and LAS. Good agreement was also found when comparing the turbulent flux of sensible heat derived from the UAS and from in-situ tower measurements at the experimental site. The occurrence of discrepancies which are mainly attributed to terrain heterogeneity are also discussed.