Numerical Study of Oceanic Boundary Layer Turbulence and Its Effect on Particles

Abstract

Turbulent flows in the oceanic surface boundary layer (OSBL) control the air-sea exchange of carbon and heat fluxes, and are an essential component in the Earth’s climate system. They also impact the vertical mixing in the upper ocean, and thus affect the transport of particles and nutrients that play a key role in marine ecosystem and biogeochemistry. However, accurate modeling of turbulent mixing remains elusive due to the inadequate knowledge of the response of turbulent flows to external forcing. In this dissertation, a high-fidelity large-eddy simulation (LES) model is applied to study the impacts of the horizontal component of Earth's rotation (fh) on the OSBL turbulence driven by different atmospheric conditions. When fh is considered, the turbulent mixing in the OSBL is dependent on the direction of surface wind stress forcing. This wind direction dependence is different at the same latitude in opposite hemispheres and is also different at different latitudes in the same hemisphere. The results suggest that wind direction is an important but overlooked parameter to the vertical mixing in the ocean, and the implementation of fh effect in vertical mixing parameterization is a potential direction for reducing the existing surface biases in the climate models. In shallow ocean, nonlinear interaction of wind-driven currents and surface gravity waves can generate a coherent water-column turbulence that enhances the vertical mixing of sediment particles. A coupled-turbulence-flocculation modeling framework is developed to study how this water-column turbulence and wave breaking impact the inter-particle interactions, and thus the size and spatial distributions of sediment particles. The results show that the water-column turbulence suspends sediment particles in the water column and organizes particles of different sizes. The modulated flocculation processes could lead to a particle size distribution that changes with depth, and a mass concentration profile that varies with the particle size. The results also show that the increase in cross-shelf sediment transport due to the presence of the water-column turbulence is comparable to that due to wave breaking under the simulated condition.