An evaluation of vertical mixing parameterization of ocean boundary layer turbulence for cohesive sediments

Abstract

Accurate parameterization for both diffusivity and turbulent kinetic energy (TKE) dissipation rate is important for the simulation of reactive tracers such as cohesive sediments. We implemented a second-order closure parameterization for mixing in ocean surface boundary layer in the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model. The parameterization is more suitable than the existing parameterizations in the COAWST model for the modeling of cohesive sediments: It includes the wave-driven Langmuir turbulent effect, a more complete pressure strain covariance parameterization in the eddy viscosity and diffusivity, and also TKE dissipation rate. Solutions using a one-dimensional configuration are compared to solutions using a three-dimensional model that simulates the ocean surface boundary layer turbulence and size distributions of flocs of different sizes. The result shows that the simulation using the newly implemented parameterization reproduces fairly well the profiles of vertical eddy viscosity, TKE dissipation rate, total mass concentration of suspended sediment, and mass averaged settling velocity in wave-driven Langmuir turbulence. The water depth dependence of floc size distribution is also reproduced in the one-dimensional model. In addition, the result based on the standard k − ω model mostly underestimates (up to ∼90%) the averaged settling velocity of suspended sediments in the water column. The result also suggests that misrepresentation of Langmuir turbulence effect in vertical mixing parameterization could cause substantial biases in the forecast/hindcast transport model for cohesive sediment.