Large Eddy Simulation of the Ocean Mixed Layer: The Effects of Wave Breaking and Langmuir Circulation

Abstract

The settling process of suspended particles in the turbulent flow was investigated by analyzing the Lagrangian motion of a large number of particles in the ocean mixed layer simulated by large eddy simulation ͑LES͒, focusing on the role of Langmuir circulation ͑LC͒. At the beginning, particles released near the surface are swept down rapidly following the downward jets in the presence of LC, contrary to the case of the horizontally uniform downward propagation in the absence of it. The strong vertical mixing by LC keeps the particle concentration almost uniform after the initial period in the presence of LC, while its mean concentration continues to decrease through particle settling. Under the influence of turbulence in the ocean mixed layer, the particle settling velocity W is always smaller than w s , and W / w s decreases with decreasing w s / u‫,ء‬ where w s is the terminal velocity of a particle in the still fluid and u‫ء‬ is the frictional velocity. The presence of LC causes a further decrease of W ultimately, and the difference between the cases with and without LC is the largest at w s / u ‫ء‬ ϳ 1. The analysis of LES data reveals that particles spend more time in upward flows and that more particles tend to accumulate in the high vorticity region in the presence of LC. It suggests that particle trapping is more likely when the length scale of vortices is larger, which may lead to the decrease of W in the presence of LC. It was also found that the preferential concentration occurs away from the regions of high horizontal vorticity and divergence when w s / u ‫ء‬ ϳ 1.

Airflow measurements at a wavy air–water interface using PIV and LIF

“…the Stokes drift, and the surface shear current, leads to the generation of Langmuir circulations, causing significant mixing of the water column (Skyllingstad and Denbo 1995;McWilliams et al 1997;Noh et al 2005;Li et al 2005;Harcourt and D'Asaro 2008; Grant and E 2009;Veron et al 2009;Kukulka et al 2010;Belcher et al 2012;D'Asaro 2014). When surface waves break, the turbulence injected into the water column also significantly enhances surface mixing Melville et al 1998;Veron and Melville 1999;Melville et al 2002;Thorpe et al 2003;Gemmrich and Farmer 2004) and leads to substantial deviations from the classical theories (Agrawal et al 1992;Thorpe 1993;Melville 1994;Anis and Moum 1995;Melville 1996;Terray et al 1996; Veron and Melville 2001) and causes significant energy dissipation (Banner et al 2014;Thomson et al 2016;Schwendeman et al 2014;Zappa et al 2016; Melville 2013, 2015).…”

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

“…Parametrization schemes might, however, be provided within the user interface (see Sect. 4.5) and have been used, e.g., by Noh et al (2004). The ocean option in its current state was recently used for simulations of the ocean mixed layer by Esau (2014), who investigated indirect air-sea interactions by means of the atmosphereocean coupling scheme that will be described in Sect.…”

“…2.8. Note that most previous PALM studies of the OML used the atmospheric code, subsequent inversion of the z axis and appropriate normalization of the results, instead of using the relatively new ocean option (e.g., Noh et al, 2004Noh et al, , 2009). …”

Wave-turbulence scaling in the ocean mixed layer

“…There has been a desire for more observations of near-surface values of due to the growing prevalence of using large eddy simulations (LES) to model the OBL (McWilliams et al, 1997;Noh et al, 2004;Grant and Belcher, 2009). The key parameter in balancing the sub-grid dynamics is and due to it not being resolved directly it is required to be parameterized.…”

“…The difficulty lies in the lack of observations of in the near surface and that without these measurements it is difficult to properly model these processes accurately (Noh et al, 2004;Grant and Belcher, 2009). In this paper we describe a set of observations taken during a research cruise in the North Atlantic with the upwardly rising Air-Sea Interaction Profiler (ASIP) microstructure instrument, an ultrasonic wave altimeter, and a high quality suite of meteorological sensors.…”