Abstract
Abstract. Solitary trees are natural land surface elements found in almost all climates, yet their influence on the surrounding airflow is poorly known. Here we use state-of-the-art, laser-based, remote sensing instruments to study the turbulent wind field in the near-wake region of a mature, open-grown oak tree. Our measurements provide for the first time a full picture of the mixing layer of high turbulence that surrounds the mean wind speed deficit. In this layer, we investigate the validity of a two-dimensional vectorial relation derived from the eddy-viscosity hypothesis, a hypothesis commonly used in modelling the turbulence transport of momentum and scalars in the atmosphere. We find that the momentum fluxes of the streamwise wind component can be adequately predicted by the transverse gradient of the mean flow. Using the mixing-length hypothesis we find that for this tree the corresponding turbulence length scale in the mixing layer can be approximated by one height-independent value. Further, the laser-based scanning technology used here was able to accurately reveal three-dimensional turbulent and spatially varying atmospheric flows over a large plane without seeding or intruding the atmospheric flow. This capability points to a new and more exact way of exploring the complex earth–atmosphere interactions.
Highlights
Solitary trees are common elements on earth’s surface, planted or naturally grown, in urban and rural landscapes as well as in tundras and grasslands
Regarding the estimation of the secondorder moments, the vertical trends of variances and covariances measured by the sonic anemometers are captured by the wind lidars
Regarding the estimation of the momentum fluxes, we find overall relative error values that are up to 128 % above the tree, but the error in the wake is limited to 20 %
Summary
Solitary trees are common elements on earth’s surface, planted or naturally grown, in urban and rural landscapes as well as in tundras and grasslands. Since trees are exceptionally efficient at extracting momentum from the wind (Lee et al, 2014; Dellwik et al, 2019), they can cause a significant effect in the near-surface atmosphere. This effect of the wind–tree interaction explains why trees are used as engineering elements in shelter belts to reduce the mean wind (Miller et al, 1974), decrease traffic noise (Kragh, 1981), improve crop productivity (Campi et al, 2009) and mitigate surface erosion (Miri et al, 2017). Despite its fundamental importance, the scientific description of trees’ interaction with the atmosphere is surrounded by large uncertainties
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