Scale‑Aware Anisotropy in Very Stable Boundary Layers: Insights from Ultra‑High‑Resolution LES

Very stable boundary layers (SBLs) exhibit weak, intermittent turbulence with strongly suppressed vertical motions. In these conditions  turbulence is highly anisotropic and varies strongly in space and time. As a result, common scaling approaches and turbulence closures that assume near‑isotropy or rely only on bulk fluxes (e.g., MOST) often fail to represent momentum and scalar transport in very stable regimes. Moreover, many Large Eddy Simulation setups and subgrid models tend to produce overly isotropic small‑scale motions, masking the true scale dependence of anisotropy.

We analyze high-resolution Large Eddy Simulation data from the psNCAR LES code simulating the GABLS1 and modified GABLES 3 case, that correspond to canonical high-Reynolds-number stably stratified boundary layers driven by constant geostrophic winds over a horizontally homogeneous surface, and two different surface cooling rates corresponding to weakly and strongly stratified turbulence. The domain size is 400 m × 400 m × 400 m with a fine grid resolution of approximately 20 cm, enabling detailed capture of turbulent structures. This spatial resolution enables us to determine the scales at which turbulence remains anisotropic, minimizing the influence of subgrid‑scale parameterizations.

We compute anisotropy from the Reynolds stress tensor and use multiresolution decompositions to examine how stratification influences the change of anisotropy with scale. These scale‑aware results are then used to compare this scalewise return to isotropy to the trajectories found in literature (Stiperski et al. 2021) and the predictions of pressure-strain interactions for SBL (Yi et al. 2025), as well as to asses the anisotropy‑aware MOST formulation at different SBL length scales.