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

[01:19 . 38]在长波长底部地形上潮汐产生的内波被辐射到内部，然后分解成小尺度，这可能是对底波混合的重要贡献。潮汐与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）近场调查的一部分，系泊仪器的最新数据表明，在相当厚的底部区域（O(250)m），根据潮汐相位，耗散和变异性水平增加。对M2潮期在粗糙坡上振荡的层状流混合特性了解甚少。采用非流体静力模型进行高精度数值模拟，以了解观测到的底部边界层行为背后的相关过程。研究了深海中起伏地形上的M2潮对海流的调制作用。在模型问题中，浅层坡度代表大尺度地形，而指定高度和节距的高斯粗糙度元代表小尺度地形。靠近底部的Brunt-Vaisala频率值随着潮汐调制的变化而变化，随着时间的推移，随着水流的加速、减速或在小尺度粗糙度上的潜在分离，分层与水流的相互作用也很有趣。与这个问题相关的物理学与内波产生问题有着根本的不同，虽然很重要，但研究得比后者少得多。大涡模拟将采用先进的数值方法对非定常的三维原始Navier-Stokes方程进行模拟，没有任何流体静力假设。所采用的子网格模型是一种采用壁层建模的动态混合模型。一个广义坐标公式允许网格跟踪底部地形，有助于保持精度。与HOME情况相关的参数将与HOME PI一起选择。得到有效边界层厚度和维萨拉频率、浮力通量、波动强度和涡动扩散率等重要非量纲参数的标度函数。更广泛的影响：拟议研究的结果将有助于理解在靠近底部的意外厚区观测到的混合现象与M2潮汐调制之间的联系，从而影响物理海洋学。鉴于我们缺乏关于在分层旋转介质中粗糙地形上振荡边界层混合的知识，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.