Turbulence Anisotropy and the Roughness Sublayer in diverse Canopy Flows: a pathway for generalization

Turbulent exchanges within the atmospheric surface layer (ASL) are influenced by the presence of canopies and terrain heterogeneity. Because the roughness sublayer (RSL) connects the canopy-terrain system to the ASL, the flow properties within the RSL are receiving significant attention. However, elementary properties of the RSL, including its thickness, remain unanswered beyond order-of-magnitude arguments. While different approaches have been developed to identify the RSL thickness, results do diverge depending on the measure used. Using Large Eddy Simulations (LES), the flow over dense vegetated canopies embedded in complex topography, dense and heterogeneous vegetated canopies on flat terrain, as well as urban canopies, are investigated. A reduced turbulent kinetic energy budget (TKE) is combined with turbulence anisotropy analysis to assess the structure and extent of the RSL. Results show that the imbalance between production and dissipation in the TKE budget is systematically linked to turbulence anisotropy, with a consistent threshold in the anisotropy invariant (yB ≈ 0.38) marking the transition from RSL to the inertial sublayer (ISL). This threshold is consistent across canopy configurations and terrain complexities, hinting at a potential pathway to generalize the identification of the RSL regardless of the canopy type or morphology. The proposed framework provides a generalizable method to identify RSL height using quantities readily derived from the Reynolds stress tensor, offering new opportunities for interpreting flux-tower measurements at the ecosystem scale and improving land–atmosphere parameterizations in Earth System Models (ESMs).