On the Determination of Effective Aerodynamic Roughness of Surfaces with Vegetation Patches

Large-eddy simulations of the flow over surfaces with alternating forest patches and clearings of different horizontal scale were performed, modelling the forest canopies as a horizontally homogeneous drag field. The objective was to extend previous works that studied the flow over sudden changes in aerodynamic roughness length occurring typically in the transition between small vegetation and forest but neglected the variations of displacement height. It was found that the internal boundary layers that formed in the transition between surface patches initially grew similarly for both the sudden changes of roughness and the alternating forest patches and clearings, but the turbulence produced at the tops of trees could break the regular growth, increasing the vertical propagation of surface heterogeneity and, consequently, the blending height. Also, the forest patches enhanced the Reynolds shear stress at the tree height over the clearings: when the energy extraction by the forest canopy ceased, the turbulent fluctuations increased and the turbulent shear production was kept high over much of the following clearing. Consequently, the Reynolds shear stress over the clearings decayed slowly, or not at all in the case of short patches. This resulted in higher average shear stress and effective aerodynamic roughness length than was the case when variations of displacement height were neglected.