SGER: Mixing in a Tidally Modulated Boundary Layer over Rough Topography

在长波长底地形上潮汐产生的内波被辐射到内部，随后分解成小尺度，可能是日周期混合的重要贡献因素。潮汐与较小尺度地形的相互作用也与近场混合有关，作为夏威夷海洋混合实验(HOME)近场调查一部分的系泊仪器的最新数据表明，在一个相当厚的海底区域，O(250)m的耗散和可变性水平增加，这取决于潮汐阶段。在粗糙的斜坡上振荡的分层流与M2潮汐周期的混合特性还知之甚少。将使用非静力模式进行高精度的数值模拟，以了解作为观测到的底部边界层行为基础的相关过程。考虑了粗糙地形上深海海流M2潮的调制作用。在模型问题中，浅坡度表示大尺度地形，而指定高度和节距的高斯粗糙度元素表示小尺度地形。底部附近的Brunt-Vaisala频率的值随潮汐调制而变化，以及随时间变化的层结与洋流在小尺度粗糙度上加速、减速或潜在分离的相互作用是有意义的。与这个问题相关的物理问题与内波产生问题有根本的不同，虽然很重要，但与后者相比，人们对它的研究要少得多。大涡模拟将在没有任何静力假设的情况下，用先进的数值方法对原始形式的非定常三维N-S方程进行。所使用的子网格模型是带有墙层模型的动态混合模型。广义坐标公式允许网格跟踪底部地形，有助于保持精度。与国内情况有关的参数将与国内PI一起选择。将得到有效边界层厚度和Vaisala频率、浮力通量、脉动强度和涡扩散系数随重要的无量纲参数变化的标度。更广泛的影响：拟议的研究结果将有助于理解在靠近海底的出人意料的厚区域观察到的混合与M2潮汐调制之间的联系，从而影响物理海洋学。鉴于我们对分层旋转介质中粗糙地形上的振荡边界层混合缺乏了解，预计这项研究也将对地球物理流体动力学做出实质性贡献。

0411938Tidally generated internal waves at long-wavelength bottom topography that are radiated into the interior to subsequently breakdown into small scales may be a significant contributor to diapycnal mixing. The interaction of the tide with smaller-scale topography is also relevant to near-field mixing as exemplified by recent data from moored instruments taken as part of the Hawaii Ocean Mixing Experiment (HOME) near-field survey that indicate increased levels of dissipation and variability in a rather thick bottom region, O(250)m, depending on the tidal phase. The mixing characteristics of a stratified current oscillating on a rough slope with M2 tidal period are poorly understood. High-accuracy, numerical simulations with a non-hydrostatic model will be conducted to understand the relevant processes underlying the observed behavior of the bottom boundary layer. The modulation by a M2 tide of a current in the deep ocean over rough topography is considered. In the model problem, a shallow slope represents the large-scale topography while Gaussian roughness elements of specified height and pitch represent the small-scale topography. The value of the Brunt-Vaisala frequency, near the bottom changes in response to the tidal modulation, and the interaction of the time-dependent stratification with the current as it accelerates, decelerates or potentially separates over the small-scale roughness is of interest. The physics relevant to this problem is fundamentally different from the internal wave generation problem and, although important, has been much less studied than the latter. The Large Eddy Simulation will be performed with advanced numerical methods for the unsteady, three-dimensional, Navier-Stokes equations in primitive form without any hydrostatic assumption. The subgrid model to be used is a dynamic mixed model with wall-layer modeling. A generalized coordinate formulation allows the grid to track the bottom topography, helpful for maintaining accuracy. Parameters relevant to the HOME situation will be chosen in conjunction with HOME PI's. The scaling of the effective boundary layer thickness and profiles of the Vaisala frequency, buoyancy flux, fluctuation intensities and eddy diffusivities as a function of important nondimensional parameters will be obtained. Broader Impacts: The results of the proposed research will help understand the link between the mixing observed in an unexpectedly thick region near the bottom and the M2 tidal modulation and thus impact physical oceanography. Given our lack of knowledge about mixing in an oscillating boundary layer over rough topography in a stratified, rotating medium, it is expected that this study will result in a substantive contribution to geophysical fluid dynamics as well.