The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime
The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime
复制标题
斜坡上湍流层状海底边界层的演化和停止：下坡状态
DOI：
--
复制
发表时间：
2019
影响因子：
3.5
通讯作者：
John R. Taylor
中科院分区：
文献类型：
--
作者：
X. Ruan;A. Thompson;John R. Taylor
绝热倾斜表面上的分层海洋底部边界层（BBL）的动力学关键取决于密度面与底部的相交。对于一个强加的顺坡流，横坡埃克曼运输平流密度表面，并产生近底地转热风切变，反对背景流。当科里奥利力和恢复浮力之间的动量平衡响应于斜坡上分层流体的位移时，出现极限情况：这被称为埃克曼制动。然而，伴随这一调整而来的动荡特征却较少受到关注。我们提出了两个估计的基础上的混合层厚度的BBL状态的特征：Ha和HL。前者表征稳定的Ekman被捕状态，后者表征再层化状态。HL的推导利用了一个新定义的斜率Obukhov长度Ls，它表征了剪切生产和跨斜率浮力平流的相对重要性。Ha的值可以与混合层H的时间演变深度相结合，形成无量纲变量H/Ha，该变量提供了不同湍流状态下BBL演变的相似性预测。长度尺度Ls也可用于获得BBL再层压时壁应力的表达式。我们验证这些关系使用一套三维大涡模拟输出。我们的结论是，BBL达到稳定的Ekman逮捕状态之前的再层化状态。计算H/Ha和H/HL的测量将提供信息的海洋BBL的发展阶段正在观察。这些诊断也可能有助于改善数值参数化的分层BBL动力学在倾斜的地形。
The dynamics of a stratified oceanic bottom boundary layer (BBL) over an insulating, sloping surface depend critically on the intersection of density surfaces with the bottom. For an imposed along-slope flow, the cross-slope Ekman transport advects density surfaces and generates a near-bottom geostrophic thermal wind shear that opposes the background flow. A limiting case occurs when a momentum balance is achieved between the Coriolis force and a restoring buoyancy force in response to the displacement of stratified fluid over the slope: this is known as Ekman arrest. However, the turbulent characteristics that accompany this adjustment have received less attention. We present two estimates to characterize the state of the BBL based on the mixed layer thickness: Ha and HL. The former characterizes the steady Ekman arrested state, and the latter characterizes a relaminarized state. The derivation of HL makes use of a newly defined slope Obukhov length Ls that characterizes the relative importance of shear production and cross-slope buoyancy advection. The value of Ha can be combined with the temporally evolving depth of the mixed layer H to form a nondimensional variable H/Ha that provides a similarity prediction of the BBL evolution across different turbulent regimes. The length scale Ls can also be used to obtain an expression for the wall stress when the BBL relaminarizes. We validate these relationships using output from a suite of three-dimensional large-eddy simulations. We conclude that the BBL reaches the relaminarized state before the steady Ekman arrested state. Calculating H/Ha and H/HL from measurements will provide information on the stage of oceanic BBL development being observed. These diagnostics may also help to improve numerical parameterizations of stratified BBL dynamics over sloping topography.
影响因子：
3.5
作者：
L. Umlauf;W. Smyth;J. Moum
通讯作者：
L. Umlauf;W. Smyth;J. Moum
影响因子：
3.5
作者：
de Lavergne, Casimir;Madec, Gurvan;Garabato, Alberto C. Naveira
通讯作者：
Garabato, Alberto C. Naveira