How does a single tree affect the roughness of a landscape: Results from a Single Tree Experiment

When looking at an agricultural landscape from above, one sees a patchwork of fields, whereas tall trees and alleys between the fields only cover a small fraction of the area. However, the trees can be prominent when observing the landscape from ground level. This view and emphasis on agricultural fields is mirrored in the way the agricultural landscape is treated in meteorological models; since the land surface is accounted for by assigning characteristic parameters to dominant land use classes from satellite imagery, the small scale tree features of the landscape are ignored. The overall aim of this study is to quantify the effect of solitary trees on the wind field and offer an estimate of how the presence of scattered trees would affect the surface roughness for a larger scale model. We present high-detail laser scans of solitary trees and discuss how they can successfully be transferred into parameterizations of the landscape in coarser resolution models. We also present wake measurements caused by solitary trees observed with both long-range and short range wind scanners, which spatially resolve the wind field. In previous work (Dellwik et al. 2019), the high resolution scans of tree geometry were used in high-resolution computational fluid dynamics (CFD) models, where the tree is parameterized as a distributed drag force. A novelty used in this study was that the total force on the tree had been experimentally assessed using strain gauges, which allowed for a much more certain assessment of the tree parameterization in the CFD model. By using the simulated wind fields from the CFD model and the total observed force on the tree in combination with basic micro-meteorological relationships, the effect of a solitary tree can be quantified in terms of changes to surface roughness.