Explicit Modeling of Near-Surface ABL Turbulence at Sub-Meter Grid Spacings via One-Way LES-to-LES Nesting

The feasibility of LES-to-LES nesting technique targeting sub-meter grid spacings is investigated. The proposed one-way nesting approach combines non-uniform vertical grid refinement and a domain vertically embedded within the parent large-eddy simulation (LES), in addition to the standard constant horizontal grid refinement. The transition and equilibration of flow solutions in sub-meter nested LES domains are systematically analyzed for idealized forcing conditions across stability regimes, and spanning a broad range of parent/nest grid refinements in both the horizontal and vertical directions. Fine-scale eddies emerge in the LES domains after a moderate fetch from the predominant inflow boundaries. In general, development fetches are delayed with increasing (Formula presented.) and distance from the surface, with cases where (Formula presented.) 10 exhibiting a more energetic quasi-equilibrium state. Spatial evolution of resolved turbulence kinetic energy (TKE) and spectra reveal two mechanisms in the transition and equilibration process. An initial development of additional small scales and a slower TKE decrease associated with a reduction in the larger-scale energy content via forward energy cascade. The specific transition mechanisms are found to be dependent on the (Formula presented.) / (Formula presented.) ratio and are impacted by atmospheric stability. The proposed scale-separation parameter, (Formula presented.), provides qualitative information about the required development fetch by combining the opposing effects of grid refinements and resolved turbulence levels in the parent domain. Larger (Formula presented.) values lead to an increased fetch, with (Formula presented.) 1.5 typically indicating TKE over prediction due to excessive scale separation between parent and nest and/or non-convergence.