Submesoscale process studies using a new variant of second order closure

Ageostrophic baroclinic instabilities of lateral density gradients in the mixed layer evolve into eddies of O(1-10km) horizontal scale, characterized by O(1) Rossby number and O(1) Richardson number. This work focuses on submesoscale process studies using a new variant of second order closure for vertical mixing. This new approach suggested by Venayagamoorthy and Stretch (2010) uses the original k-epsilon model for the turbulent viscosity, but poses a turbulent Prandtl number model in an irreversible mixing framework to derive the diffusivity, providing a less heuristic parameterization compared to previous models. This new variant agrees well with previous closure models for idealized studies of entrainment mixing with and without a diurnal cycle. Tests of this model with observational datasets show high near-inertial shear below the mixed-layer and absence of shear within the mixed-layer, in agreement with earlier studies. This variant has subsequently been implemented in a three-dimensional submesoscale resolving process model. Model simulations initialized with an idealized wind forced frontal domain show the evolution of submesoscale eddies, restratifying buoyancy flux and enhanced dissipation at the front. Zonally averaged eddy kinetic energy budget at the frontal region shows a balance between the ageostrophic shear produced by wind forcing, and subgrid dissipation. The magnitude of near-surface subgrid dissipation is consistent with Monin-Obukhov scaling, as with a previous anisotropic Smagorinsky model. In June 2011 a large set of observations were made as part of the Directed Research Initiative of Scalable Lateral Mixing and Coherent Turbulence - LATMIX 2011, funded primarily by the office of Naval Research. We report on tracer simulations inspired by the LATMIX 2011 observations to help understand the mechanisms for lateral and vertical mixing.