Most submesoscale motions-fronts, eddies, filaments, and even large internal waves-are sufficiently rapidly rotating and stratified as to be strongly influenced by potential vorticity dynamics. Below the Ozmidov scale where turbulence overturns and isotropizes, potential vorticity is not commonly considered. Here, it is shown that in Large Eddy Simulations, the velocity gradients, buoyancy gradients, and potential vorticity are strongly influenced by grid-scale processes. Grid-scale processes in Large Eddy Simulations, as opposed to those in Direct Numerical Simulations, imply that spuriously noisy potential vorticity variance will become increasingly dominant as resolution increases-analogous to ultraviolet catastrophe. A solution, the prefiltered potential vorticity, is shown to be effective in linking the potential vorticity dynamics of the submesoscale to the nearly-isotropic turbulent fluxes beyond the Ozmidov scale, and a derivation is provided for a set of closed conservation equations for use in interpreting potential vorticity dynamics in Large Eddy Simulations. This diagnostic approach is exceptional in that Large Eddy Simulation analysis and hydrostatic ocean modeling with parameterized turbulence analysis are harmonized. Plain Language Summary In the study of geophysical fluid dynamics, such as oceans and atmosphere, it is common to use models such as Large Eddy Simulations (LES) that favor large-scale dynamics over dynamics near the grid scale, which are approximated using turbulence closures. So, if parameterizations of small-scale processes hidden below or near the model grid scale dominate, caution must be taken in interpretation of results. Potential vorticity combines rotation and stratification into one variable which offers key insights into dynamical properties for atmospheric and oceanic flows where rotation and stratification are important (i.e., large scales). However, as small scales are approached and turbulent mixing dominates the flow, potential vorticity transitions to being strongly influenced by the smallest scales, which in LES are the least reliable, unresolved, or heavily approximated processes. In this study, we present an accurate diagnosis method for potential vorticity dynamics in such models, by treating all scales associated with turbulent mixing separately, and find the corresponding larger scale potential vorticity and its governing equations.
Current submesoscale restratification parameterizations, which help set mixed layer depth in global climate models, depend on a simplistic scaling of frontal width shown to be unreliable in several circumstances. Observations and theory indicate that frontogenesis is common, but stable frontal widths arise in the presence of turbulence and instabilities that participate in keeping fronts at the scale observed, the arrested scale. Here we propose a new scaling law for arrested frontal width as a function of turbulent fluxes via the Turbulent Thermal Wind (TTW) balance. A variety of Large Eddy Simulations (LES) of strain-induced fronts and TTW-induced filaments are used to evaluate this scaling. Frontal width given by boundary layer parameters drawn from observations in the General Ocean Turbulence Model (GOTM) and are found qualitatively consistent with the observed range in regions of active submesoscales. The new arrested front scaling is used to modify the mixed layer eddy restratification parameterization commonly used in coarse resolution climate models. Results in CESM-POP2 reveal the climate model’s sensitivity to the parameterization update and changes in model biases. A comprehensive multi-model study is in planning for further testing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.