Large-eddy simulation of the roughness sublayer within and above forest canopies and its transition to the outer regions of the atmospheric boundary layers

A large-eddy simulation of an Ekman boundary layer with the lowest 20 m occupied by a model forest is performed. The simulation has a domain size much greater than most previous LES of canopy airflows, especially in the vertical direction. Single-point integral turbulent statistics illustrate that the depth of the roughness sublayer is about twice the canopy height. Above the roughness layer, a thin logarithmic layer appears to exist between 2.5-3 times canopy height. Two-point correlations indicate that the vertical extent of coherent structure centered near the canopy top is up to about 2-3 times the canopy height depending on whether a contour level of 0.2 or 0.1 is used (except for the two-point correlations of pressure perturbations). This is not significantly different from previous LES of canopy flow in neutral conditions. However, the horizontal extent of the coherent structures shown in the two-point correlations is appreciably greater than those in previous LES of canopy flows with smaller domain size. The effects of more extended horizontal domain, boundary layer depth and stability and canopy morphology on the roughness sublayer are still on-going investigation.