Dispersion of Buoyant and Sinking Particles in a Simulated Wind- and Wave-Driven Turbulent Coastal Ocean

In shallow coastal oceans, turbulent flows driven by surface winds and waves and constrained by a solid bottom disperse particles. This work examines the mechanisms driving horizontal and vertical dispersion of buoyant and sinking particles for times much greater than turbulent integral time scales. Turbulent fields are modeled using a wind-stress driven large eddy simulation (LES), incorporating wave-driven Langmuir turbulence, surface breaking wave turbulent kinetic energy inputs, and a solid bottom boundary. A Lagrangian stochastic model is paired to the LES to incorporate Lagrangian particle tracking. Within a subset of intermediate buoyant rise velocities, particles experience synergistic vertical mixing in which breaking waves (BW) inject particles into Langmuir downwelling velocities sufficient to drive deep mixing. Along-wind dispersion is controlled by vertical shear in mean along-wind velocities. Wind and bottom friction-driven vertical shear enhances dispersion of buoyant and sinking particles, while energetic turbulent mixing, such as from BW, dampens shear dispersion. Strongly rising and sinking particles trapped at the ocean surface and bottom, respectively, experience no vertical shear, resulting in low rates of along-wind dispersion. Crosswind dispersion is shaped by particle advection in wind-aligned fields of counter-rotating Langmuir and Couette roll cells. Langmuir cells enhance crosswind dispersion in neutrally to intermediately buoyant particles through enhanced cell hopping. Surface trapping restricts particles to Langmuir convergence regions, strongly inhibiting crosswind dispersion. In shallow coastal systems, particle dispersion depends heavily on particle buoyancy and wave-dependent turbulent effects.