Large-Eddy Simulation of Reynolds Stress Budgets in and Above Forests in Neutral Atmospheric Boundary Layers

Large-eddy simulations (LESs) of inversion-capped neutral atmospheric boundary layers (ABLs) are augmented to earlier small-domain LESs of a sparse forest and field observation to evaluate the budgets of all non-negligible resolved-scale Reynolds stress components. The focus is on the atmospheric surface layer comprised of the roughness sublayer (RSL) in and above horizontally homogeneous forests and the inertial sublayer (ISL) above the RSL over flat terrain. The greater LES domain and ABL depths result in greater depths of both the RSL and the ISL. A key result is that in the upper portions of the canopy and above, pressure redistribution is a major sink of normal stress in the horizontal direction with mean shear production as a major source, whereas in the horizontal direction absent of mean shear production and in the vertical direction, pressure redistributions are major sources of normal stresses. In the lower portions of the canopy where mean shear production and turbulent transport are much reduced, pressure redistributions are major sources of horizontal velocity variances but a major sink of vertical velocity variance. Pressure transport is a greater source of vertical velocity variance than turbulent transport from the ground level to just under the treetops where it transitions to a major sink up to about 1.5 times the canopy height. This greater significance of pressure transport over turbulent transport increases with increased vegetation area index (VAI). The impact of increased value of geostrophic wind speed is negligible compared to that of increased value of VAI on enhancing normalized budget terms in the vicinity of treetops.