A new physically-motivated vertical mixing scheme for ocean surface boundary layer turbulence

In this study, we present a physically-motivated Assumed-Distribution Higher-Order Closure (ADC) scheme to parameterize small-scale vertical mixing in the ocean surface boundary layer (OSBL). The ADC is a hybrid mass-flux and high-order closure scheme that achieves turbulence closure by reconstructing all the required second-order and higher-order moments exactly from a subset of moments that are evolved prognostically. In addition, this ADC parameterization has energetic constraints and includes non-local fluxes, which are a significant source of vertical mixing in the upper ocean boundary layer. We have implemented the ADC scheme within the Model for Prediction Across Scales-Ocean (MPAS-Ocean), the ocean component of the U.S. Department of Energy's Energy Exascale Earth System Model (E3SM). We evaluated this scheme against three dimensional large-eddy simulations (LES) using a single-column model formulation for general ocean-relevant forcing. Results show that the new scheme can efficiently simulate upper ocean vertical mixing produced by various processes including surface wave-, buoyancy-, and wind-driven mixing.