Abstract
Abstract. High-resolution thermal infrared (TIR) imaging is opening up new vistas in biosphere–atmosphere heat exchange studies. The rapidly developing unmanned aerial systems (UASs) and specially designed cameras offer opportunities for TIR survey with increasingly high resolution, reduced geometric and radiometric noise, and prolonged flight times. A state-of-the-art science platform is assembled using a Matrice 210 V2 drone equipped with a Zenmuse XT2 thermal camera and deployed over a pristine boreal peatland with the aim of testing its performance in a heterogeneous sedge-fen ecosystem. The study utilizes the capability of the UAS platform to hover for prolonged times (about 20 min) at a height of 500 m a.g.l. while recording high frame rate (30 Hz) TIR videos of an area of ca. 430 × 340 m. A methodology is developed to derive thermal signatures of near-ground coherent turbulent structures impinging on the land surface, surface temperature spectra, and heat fluxes from the retrieved videos. The size, orientation, and movement of the coherent structures are computed from the surface temperature maps, and their dependency on atmospheric conditions is examined. A range of spectral and wavelet-based approaches are used to infer the properties of the dominant turbulent scene structures. A ground-based eddy-covariance system and an in situ meteorological setup are used for reference.
Highlights
One of the long-standing problems in turbulence research, turbulence in the planetary boundary layer (PBL), is the heat transfer between rough surfaces and the turbulent flow aloft
While the stability parameter zL−O1 estimated from 3 m EC data pointed at near-neutrality on 28 August, the higher wind speed and friction velocity indicate a predominantly mechanical or shear-induced PBL turbulence production, as opposed to 6 August when the PBL turbulence was more buoyancy produced
The mean wind speed and direction obtained by thermal image velocimetry (TIV) are similar to the anemometric observations
Summary
One of the long-standing problems in turbulence research, turbulence in the planetary boundary layer (PBL), is the heat transfer between rough surfaces and the turbulent flow aloft. This study focuses primarily on the properties of large coherent turbulent structures, or dominant eddies as termed by Taylor (1958) He was the first to draw attention to the regular features in air temperature time series, which Priestley (1959) later linked to the thermals generated by surface roughness and buoyancy. Air parcels residing near the ground attain buoyancy upon receipt of heat from the ground and rise up to become replaced by cooler air parcels descending from above in a cyclical manner Such ascending and descending air parcels can reach the size of the entire boundary layer, i.e., hundreds to thousands of meters across (Kaimal and Businger, 1970; Kaimal et al, 1976)
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