Evaluation of 3D Subgrid-scale Fluxes in Kilometer-scale Simulations of an Idealized Squall Line

Subgrid-scale (SGS) turbulent mixing is essential to convection-permitting simulations where turbulent fluxes are partially resolved and partially subgrid-scale. This study investigates the characteristics of the three-dimensional (3D) SGS fluxes of an idealized squall line in a weak sheared environment. The 3D SGS fluxes on kilometer-scale grids are obtained by coarse-graining a benchmark large-eddy simulation (LES) conducted with the Advanced Regional Prediction System model. Counter-gradient (CG) transport is found in both horizontal and vertical SGS fluxes in the updraft region, which results from nonlocal transport associated with the tilted updraft. Using moist-conserved variables for the identification of CG fluxes, the spatial distribution and the occurrence rates of the CG and the down-gradient fluxes are investigated. A scale-similarity (Hgrad) and a conventional gradient-diffusion (TKE) SGS closures are then evaluated at kilometer-scale resolutions against the LES. Both the offline evaluation and the online simulations demonstrate improvements of the Hgrad closure over the TKE closure, mostly due to the former’s ability to represent CG fluxes. Through sensitivity experiments, we investigate the role of horizontal flux parameterization in predicting the structure and intensity of deep convection with tilted updraft.