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arXiv 2103.14714physics.ao-phphysics.flu-dyn
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沿海海洋中的重叠边界层

Overlapping boundary layers in coastal oceans

Chao Yan, James C. McWilliams, Marcelo Chamecki

摘要:

由于海底相互作用的存在,近岸区域的边界层湍流不同于深海区域。本文采用大涡模拟方法,通过数值求解波浪平均方程,研究了有限水深近岸海洋中表层和底层边界层的合并问题。海洋流体是由风应力、表面波和存在稳定垂直层结的恒定电流的联合作用驱动的。产生的流动由两个重叠的边界层组成,即表面和底部边界层,由内部分层分离。重叠边界层的发展经历了快速加深、振荡平衡和迅速合并三个阶段,其间有两个转捩。合并前,在分层层中观察到内波,内波主要由表面边界层中的朗缪尔湍流激发。这些波对底部产生的湍流产生明显的调制,促进表面和底部边界层之间的相互作用。合并后,原本局限于表层的朗缪尔环流被发现在尺寸上增长并向下延伸到海底(即使表面波感觉不到底部),让人想起组织良好的朗缪尔超级单体。这些全深度的朗缪尔环流促进了垂直混合并增强了底部剪切,导致垂直柱中湍流水平的显著增强。

英文摘要:

Boundary layer turbulence in coastal regions differs from that in deep ocean because of bottom interactions. In this paper, we focus on the merging of surface and bottom boundary layers in a finite-depth coastal ocean by numerically solving the wave-averaged equations using a large eddy simulation method. The ocean fluid is driven by combined effects of wind stress, surface wave, and a steady current in the presence of stable vertical stratification. The resulting flow consists of two overlapping boundary layers, i.e. surface and bottom boundary layers, separated by an interior stratification. The overlapping boundary layers evolve through three phases, i.e. a rapid deepening, an oscillatory equilibrium and a prompt merger, separated by two transitions. Before the merger, internal waves are observed in the stratified layer, and they are excited mainly by Langmuir turbulence in the surface boundary layer. These waves induce a clear modulation on the bottom-generated turbulence, facilitating the interaction between the surface and bottom boundary layers. After the merger, the Langmuir circulations originally confined to the surface layer are found to grow in size and extend down to the sea bottom (even though the surface waves do not feel the bottom), reminiscent of the well-organized Langmuir supercells. These full-depth Langmuir circulations promote the vertical mixing and enhance the bottom shear, leading to a significant enhancement of turbulence levels in the vertical column.

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