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Anisotropy of Langmuir turbulence and the Langmuir-enhanced mixed layer entrainment
Physical Review Fluids ( IF 2.7 ) Pub Date : 2020-01-30 , DOI: 10.1103/physrevfluids.5.013803
Qing Li , Baylor Fox-Kemper

The anisotropy and structure of turbulence simulated by large-eddy simulations with and without Stokes-drift forcing are analyzed, with an emphasis on the linkage between the distinctive structure of Langmuir turbulence near the surface where cellular vortices aligned with the wind and wave propagation direction are apparent and the Langmuir-enhanced mixed layer entrainment at the base of the ocean surface boundary layer (OSBL) where turbulent structures differ. The tensor invariants of the Reynolds stresses, the variance of vertical velocity and buoyancy, and the velocity gradient statistics are used to categorize turbulence structures as a function of depth, including an extension of the barycentric map to show the direction as well as the magnitude of turbulence anisotropy and a vector-invariant extension of the Okubo-Weiss parameter. The extended anisotropic barycentric map and the velocity gradient statistics are demonstrated to be useful, providing compact information of the anisotropy, orientation, and structure of turbulent flows. It is found that the distinctive anisotropy and structures of Langmuir turbulence are quickly lost below regions where Stokes drift shear is significant and vortices are apparent, consistent with past observations and model results. As a result, the turbulent structures near the base of the OSBL are not significantly affected by the presence of Stokes drift above but are instead dominated by local Eulerian shear, except in one important manner. Langmuir turbulence does affect the mixed layer entrainment by providing extra available turbulent kinetic energy (TKE) via enhanced near-surface TKE production and higher vertical TKE transport energizing the turbulent structures near the base of the OSBL. The additional TKE is utilized by structures similar to those that exist without Stokes drift forcing in terms of anisotropy of their Reynolds stresses, but they are more energetic because of the Langmuir turbulence. Thus, parametrizing the major aspects of Langmuir turbulence on entrainment at the base of the OSBL can be incorporated through enhancing available energy without other modifications.

中文翻译:

Langmuir湍流的各向异性和Langmuir增强混合层夹带

分析了具有和不具有斯托克斯漂移强迫的大涡模拟所模拟的湍流的各向异性和结构,并着重强调了在表面涡旋与风成一直线与波传播方向一致的表面附近的朗缪尔湍流的独特结构之间的联系。在湍流结构不同的海面边界层(OSBL)的底部存在明显的朗格缪尔增强混合层夹带。雷诺应力的张量不变量,垂直速度和浮力的变化以及速度梯度统计数据用于根据深度对湍流结构进行分类,包括重心图的扩展以显示方向和强度Okubo-Weiss参数的湍流各向异性和矢量不变的扩展。扩展的各向异性重心图和速度梯度统计数据被证明是有用的,它提供了各向异性,湍流和湍流结构的紧凑信息。结果发现,在斯托克斯漂移剪切力明显且涡旋明显的区域下方,朗格缪尔湍流的独特各向异性和结构迅速消失,这与过去的观测结果和模型结果一致。结果,在OSBL底部附近的湍流结构不受以上斯托克斯漂移的影响而受到显着影响,而是由局部欧拉剪切控制,除非以一种重要的方式。朗缪尔湍流的确通过增加近地表面TKE的产生和更高的垂直TKE输送来提供额外的可用湍动能(TKE),从而影响混合层夹带,从而为OSBL底部附近的湍流结构提供能量。就其雷诺应力的各向异性而言,类似于在没有Stokes漂移强迫的情况下存在的结构,可以利用额外的TKE,但由于Langmuir湍流,它们的能量更高。因此,可以通过在不进行其他修改的情况下提高可用能量来合并对兰格缪尔湍流在OSBL底部夹带的主要方面进行参数化的方法。就其雷诺应力的各向异性而言,类似于在没有Stokes漂移强迫的情况下存在的结构,可以利用额外的TKE,但由于Langmuir湍流,它们的能量更高。因此,可以通过在不进行其他修改的情况下提高可用能量来合并对兰格缪尔湍流在OSBL底部夹带的主要方面进行参数化的方法。就其雷诺应力的各向异性而言,类似于在没有Stokes漂移强迫的情况下存在的结构,可以利用额外的TKE,但由于Langmuir湍流,它们的能量更高。因此,可以通过在不进行其他修改的情况下提高可用能量来合并对兰格缪尔湍流在OSBL底部夹带的主要方面进行参数化的方法。
更新日期:2020-01-31
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