5:15 PM - 6:45 PM
[MIS09-P12] The mixed layer depth dependence of the horizontal transport of floating materials simulated with LES
Keywords:microplastics, drift simulation, large eddy simulation
Marine plastic pollution has become a global concern in recent years, with ongoing efforts to address this issue. The drift simulation of plastic debris is an essential tool for understanding and predicting their global distribution. However, the drift simulation still contains large uncertainty, majorly due to the uncertainty in the drift modeling. To improve its accuracy, a deeper understanding and better representation of the subgrid-scale (SGS) phenomena are essential because they affect the horizontal transport and dispersion of the floating materials. Some studies using the large-eddy simulation (LES) of ocean surface mixed layer suggest the importance of including the characteristics of the mixed layer such as the wave-induced mixing associated with the Stokes drift into horizontal drift parameterizations. Therefore, it is well expected that other mixed layer features such as mixed layer depths, Coriolis coefficient, sea surface heating/cooling, and wind speed, also affect the horizontal transport of floating materials. This study aims to elucidate the impact of mixed layer conditions on the horizontal transport of floating debris using ocean surface LES, with a particular focus on the influence of the mixed layer depths.
Using the nonhydrostatic ocean model KINACO, mixed layer simulations with five different initial mixed layer depths are conducted in a rectangular domain with periodic horizontal boundaries. There, Lagrangian particles with eight different rising speeds are released, and each particle’s total horizontal displacement is evaluated by counting the number of times that each particle crosses the horizontal boundaries. In addition, the mean horizontal drift evaluated as the ensemble average of the actual particle displacement is compared with two approximate formulae, namely, mean horizontal velocity weighted with particle concentration profile and mixed-layer-averaged horizontal velocity.
Results showed that the surface Lagrangian velocity directed downwind and rotated to the right was greater with shallower mixed layer depths. The vertical velocity variance increased with deeper mixed layer depths. Horizontal particle dispersion increased with shallower mixed layer depths, particularly for the particles with higher rising speeds, likely due to the influence of downwind shear dispersion. The mean horizontal drift increased with shallower mixed layer depths and higher rising speeds, indicating the importance of horizontal advection of near-surface particles caused by the fast surface currents. Furthermore, mean horizontal velocity weighted with the particle concentrations reproduced dependencies of actual particle motions on mixed layer depths and rising speeds. On the other hand, mixed-layer-averaged velocity underestimated the mean horizontal drift of the particles with higher rising speeds.
Using the nonhydrostatic ocean model KINACO, mixed layer simulations with five different initial mixed layer depths are conducted in a rectangular domain with periodic horizontal boundaries. There, Lagrangian particles with eight different rising speeds are released, and each particle’s total horizontal displacement is evaluated by counting the number of times that each particle crosses the horizontal boundaries. In addition, the mean horizontal drift evaluated as the ensemble average of the actual particle displacement is compared with two approximate formulae, namely, mean horizontal velocity weighted with particle concentration profile and mixed-layer-averaged horizontal velocity.
Results showed that the surface Lagrangian velocity directed downwind and rotated to the right was greater with shallower mixed layer depths. The vertical velocity variance increased with deeper mixed layer depths. Horizontal particle dispersion increased with shallower mixed layer depths, particularly for the particles with higher rising speeds, likely due to the influence of downwind shear dispersion. The mean horizontal drift increased with shallower mixed layer depths and higher rising speeds, indicating the importance of horizontal advection of near-surface particles caused by the fast surface currents. Furthermore, mean horizontal velocity weighted with the particle concentrations reproduced dependencies of actual particle motions on mixed layer depths and rising speeds. On the other hand, mixed-layer-averaged velocity underestimated the mean horizontal drift of the particles with higher rising speeds.