Turbulent processes in the ocean surface boundary layer (OSBL) play a key role in weather and climate systems. This study explores a Lagrangian analysis of wave-driven OSBL turbulence, based on a large-eddy simulation (LES) model coupled to a Lagrangian stochastic model (LSM). Langmuir turbulence (LT) is captured by Craik–Leibovich wave forcing that generates LT through the Craik–Leibovich type 2 (CL2) mechanism. Breaking wave (BW) effects are modeled by a surface turbulent kinetic energy flux that is constrained by wind energy input to surface waves. Unresolved LES subgrid-scale (SGS) motions are simulated with the LSM to be energetically consistent with the SGS model of the LES. With LT, Lagrangian autocorrelations of velocities reveal three distinct turbulent time scales: an integral, a dispersive mixing, and a coherent structure time. Coherent structures due to LT result in relatively narrow peaks of Lagrangian frequency velocity spectra. With and without waves, the high-frequency spectral tail is consistent with expectations for the inertial subrange, but BWs substantially increase spectral levels at high frequencies. Consistently, over short times, particle-pair dispersion results agree with the Richardson–Obukhov law, and near-surface dispersion is significantly enhanced because of BWs. Over longer times, our dispersion results are consistent with Taylor dispersion. In this case, turbulent diffusivities are substantially larger with LT in the crosswind direction, but reduced in the along-wind direction because of enhanced turbulent transport by LT that reduces mean Eulerian shear. Our results indicate that the Lagrangian analysis framework is effective and physically intuitive to characterize OSBL turbulence.
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.more » « less
- Award ID(s):
- NSF-PAR ID:
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Medium: X
- Sponsoring Org:
- National Science Foundation
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