Dispersion of buoyant Lagrangian particles in the wave-driven ocean surface boundary layer
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Upper ocean turbulence, generated by wind and wave forcing, directly controls
air-sea exchange processes and the dispersion of material within the ocean surface
boundary layer (OSBL). This study investigates the dispersion and transport of
buoyant material, such as seaweed, phytoplankton, oil, and plastics, within the OSBL
for varying buoyant rise velocities and wave conditions. Wave conditions studied
include: shear turbulence, breaking wave (BW) effects, and Langmuir turbulence
(LT). Breaking surface gravity waves transfer turbulent kinetic energy (TKE) into the
ocean and result in enhanced TKE dissipation rates and mixing within a near-surface
region. LT, captured by the Craik-Leibovich vortex force and other wave terms,
results from interactions between the wave-driven Stokes drift and the turbulent
current. LT is characterized by counter-rotating, near-surface vortices, which are a key
for horizontal organization and submergence of buoyant particles. To model buoyant
tracers in the turbulent OSBL, we employ a Lagrangian approach by tracking buoyant
particles within a simulated OSBL flow field. The flow simulations are based on a
large eddy simulation (LES) model coupled to a Lagrangian stochastic model, which
captures particle velocities not resolved by the LES. Particle clouds are released at
different vertical and horizontal positions and their dispersion characteristics
quantified with probability density functions (e.g., concentration profiles) and the
mean squared distance of particle pairs. In particular, we determine horizontal
turbulent dispersion coefficients for dispersion times much larger than turbulent
integral times. The initial dispersion of particle clouds depends on the local TKE
dissipation rate and is nearly independent of buoyant rise velocity, consistent with the
expected behavior for the inertial subrange. Enhanced TKE levels due to BW
substantially increase initial dispersion rates. For longer time scales, both mean
currents and turbulent eddies critically drive dispersion of buoyant particles within the
OSBL. For small buoyant rise velocities, particle concentrations are transported
vertically by turbulent eddies in all cases. Under shear turbulence conditions, sheared
mean currents differentially advect particle clouds with respect to depth, resulting in
large turbulent diffusion coefficients for cases without LT. In contrast, enhanced
vertical mixing due to Langmuir turbulence homogenizes currents with respect to
depth, decreasing shear dispersion and, consequentially, turbulent diffusion
coefficients for small buoyant rise velocities. When buoyant rise velocity is increased,
small-scale shear and breaking wave turbulence are unable to efficiently submerge
particle concentrations. This results in surface trapping of highly buoyant particles and
significantly reduces shear dispersion and turbulent diffusion coefficients. Large
Langmuir circulations, however, are still able to submerge highly buoyant particle
concentrations, increasing horizontal dispersion. Results of this study indicate that
dispersion of particles is highly dependent on both buoyant rise velocity and wave
conditions. Therefore, both buoyant rise velocity and wave effects must be considered
when modeling the transport of buoyant material within the OSBL.
Description
Keywords
Applied sciences, Physical sciences, Boundary layer, Buoyant particles, Lagrangian particle dispersion