Influences of tree-sway on canopy roughness sublayer turbulence: a coupled large-eddy simulation and tree-sway model

Previous field observations and modeling work have shown that honami (cereal waves) could play an important role in turbulence dynamics in the canopy roughness sublayer (CRSL) within and above short crops. However, the sway-motions of trees have not been formally considered in most turbulence models for airflows in and above forest canopies, including large-eddy simulation (LES). Thus, the aerodynamic interactions between group tree-sway and coherent gust in the CRSL in and above forest canopies are largely unknown. The focus of this presentation is to examine the influences of tree-sway on CRSL turbulence characteristics by comparing two LES runs, one of which treats the trees as rigid and motionless and the other explicitly solves the tree-sway motions of individual trees in a horizontal array (250 x 250 trees). Preliminary results show that tree-sway motions reduce mean wind speed and increase mean wind directional shear inside the forest, increase vertical mean wind shear above the forest, reduce tangential Reynolds stress and turbulent kinetic energy both inside and above the forest. The spatial patterns of group tree-sway are also shown to closely correlate to the spatial patterns of the coherent structures (sweep/ejection) in the CRSL. A range of single-point and two-point statistics will be compared, as well as the budgets of momentum, turbulence kinetic energy and Reynolds stress.