Wind loading on trees across a forest edge: A large eddy simulation

Wind loadings on trees, quantified by wind moment, are studied at a forest edge by employing a large eddy simulation. Trees at the forest edge are subject to large mean values of wind moment. If the relative magnitude of wind moment is concerned, however, wind moment appears to be more violent in the equilibrium region far downstream of the forest edge. This is explained by that wind moments at locations further than eight tree heights downwind of the edge are characterized by larger maximum-to-mean ratios, greater intensities of fluctuation and skewness, and higher frequency of extreme value than in the edge zone. These distinct spatial features between the absolute magnitude and relative magnitude of wind moment have different implications in regard to stand vulnerabilities between young and old forests. Two approaches are used to investigate the controlling mechanisms for wind moment: (1) cross-correlation analysis and (2) comparison of extreme values in wind moment to those in flow-driving terms. The results demonstrate that pressure gradient force and streamwise advection of upstream momentum are highly correlated with, and likely induce the extreme values in wind moment at the forest edge. In the equilibrium region, however, intermittent impulsive downward momentum fluxes associated with the passage of coherent structures are the ultimate mechanism in causing extremes in wind moment.