Large-Eddy Simulation of the Roughness Sublayer within and above Forest Canopies and Its Transition to the Outer Regions of the Atmospheric Boundary Layers

Most previous LES work of airflow within and above forest canopies were limited in both horizontal and vertical domain sizes. Questions remain on the validity or representation of the simulated flow fields. On the other hand, field tower-based observations are also often limited within 2-3 times canopy height. It is presently unclear how the depth of the atmospheric boundary layer (ABL) and stability and canopy morphology may influence the depth of the roughness sublayer, the characteristics of the coherent structures in the roughness sublayer and their interactions with turbulence in the outer regions of the ABL. A set of high-resolution LES runs with realistic ABL depths and sufficient within-canopy resolution are performed for neutral, unstable and slightly stable conditions. This presentation will examine differences in the vertical profiles of single-point turbulence statistics (mean to higher-order moments, spectra and cospectra, integral length and time scales) and budget analyses and evaluation of turbulent closure schemes for each of the cases. The aim is to identify the depth of the roughness sublayer and how it varies from case to case, by comparing against well-known similarity formulations for the surface layer and the mixing layer. Selected instantaneous flow fields and two-point correlation may also be used to illustrate eddy structures in the simulated flows.