What we have learned so far from SwayLES: A Coupled 3-D Large-Eddy-Simulation Tree-Sway Model?

One of the original motivations of this work was to improve our understanding and modeling of wind damages as one of the major disturbances to forest ecosystems in a changing climate. A coupled 3-D Large-Eddy-Simulation-Tree-Sway-Model (SwayLES) was developed to investigate the aerodynamic interactions between group tree-sways and coherent gusts in the canopy roughness sublayer (CRSL) in and above forest canopies. The fully (two-way) coupled version of SwayLES was first applied to a dense and a sparse model forests (vegetation area index VAI = 6.5 and 2) driven by two different horizontal pressure gradients equivalent to geostrophic winds of 40 m/s and 20 m/s in neutral stratification. A companion four LES runs in which the trees do not sway were performed to examine the effects of tree-sway on turbulence characteristics in the RSL. It was found that the impacts of tree-sways on turbulence in the CRSL is not as significant as those due to the large difference in VAI. In order to conduct a systematic investigation of the effects of tree mechanical properties on both tree-sway motions and airflow characteristics, we had to use the one-way version of SwayLES due to computational constraints, in which tree-sways were driven by 10 Hz time series of 3-D turbulent wind field in the CRSL saved from the four fully-coupled SwayLES runs. This allowed the completion of a large set of sensitivity runs with greatly varying elasticity, damping coefficient and tree mass. Despite the one-way nature of this approach, the potential impacts of tree-sways on airflow may still be assessed by evaluating differences in the drag force on the airflows. Most of the results from these sensitivity runs have not been reported previously, the highlights of which will be a focus in this presentation. These includes the impacts of tree-sway mechanical properties on transient tree-sway motions and associated forces during coherent gusts, which have distinctly different spatial and temporal scales in and above the dense and the spare forest, as well as significantly different intensity between the two geostrophic winds.