Collaborative Research: Internal waves impinging on near-critical slopes: multiscale numerical quantification of localized mixing and exchange with the interior

该项目将对入射到倾斜底部的内波和潮汐驱动的边界过程进行数值研究。特别是，该项目将对以下假设进行定量评估：内波在临界和近临界坡度处的非线性破裂导致混合流体，然后沿沿着等密度线向内部扩散，这是一个对诊断全球海洋环流具有根本重要性的问题。有间接支持这一假设，通过在近临界斜坡附近的中间云状层的观测，以及通过实验室实验的直接证据，虽然在低雷诺数。准确估计当地的耗散率和湍流输运系数在这些？热点？也将由模拟提供。通过实地观察有越来越多的证据表明，海底边界过程对于斜坡和内陆之间的流体交换、海洋中的能量通道、沉积物的悬浮和沉降以及当地的生物生产力都很重要。因此，有必要提高我们的理解和我们的能力，数值预测的内波/边界相互作用的问题和多尺度的方法，具有潜在的变革性的结果，将桥梁的大内波尺度的底部边界层（BBL）。由于入射波场和湍流之间的尺度差异、预先存在的边界层的高雷诺数以及在临界坡度情况下形成的边界层，以及当前海洋模型中用于稳定性的高数值耗散，到目前为止还不可能对底部混合过程进行真实的模拟。尺度分离的问题是通过一个新的层次使用的分析或数值解耦合到大涡模拟（LES）的底部混合的大规模问题在这里处理。为了在没有解析理论的情况下提供大尺度场，我们将采用一种新的基于自适应网格细化（AMR）的斜压模式。一个现实的描述小规模的过程将在这里获得采用LES与一个复杂的亚网格模型。该子网格模型具有尺度相似性分量以解释各向同性、惯性范围行为的偏差，它不是过度耗散的，它不需要可调系数，并且它具有近壁分量以解释边界层粗糙度尺度处的湍流。AMR和LES将双向耦合。AMR（需要时）将驱动LES，而后者将为前者提供近似的边界条件，允许边界混合进入内部。模拟将分析BBL属性，定相的本地混合率，横向扩散的BL混合流体，和非线性散射波的特性。将提取入射波的属性和倾斜角的函数关系。将与实验室和现场数据进行比较。区域或全球一级的大尺度数值模式被要求提供越来越精确的海洋对气候变化影响的评估，这取决于边界通量的参数化。这里提出的AMR和LES模拟将允许仔细评估的底部湍流动力学，然后可以用来开发一个准确的代表性的大尺度模型的底部混合组件。沿斜坡的沿着混合影响到陆架边缘/斜坡区的若干特征，而这些特征又控制着边界与公海之间营养物和污染物的交换。在更广泛的视野中，混合在陆棚斜坡上的热点是一个关键成分的能量平衡的内部波场。两名博士生将接受海洋学、工程学和计算科学领域的培训。本科生将有机会参与研究。

The project will pursue a numerical investigation of boundary processes driven by internal waves and tides incident on a sloping bottom. In particular, the project will give a quantitative assessment of the hypothesis that nonlinear breakdown of internal waves at critical and near-critical slopes results in mixed fluid that then disperses along isopycnals to the interior, a problem of fundamental importance to diagnose the global ocean circulation. There is indirect support of this hypothesis through observations of intermediate nepheloid layers in the vicinity of near-critical slopes as well as direct evidence through laboratory experiments, albeit at low Reynolds number. Accurate estimates of local dissipation rates and turbulent transport coefficients at these ?hot spots? will also be provided by the simulations. There is mounting evidence through field observations that bottom boundary processes are important to the exchange of fluid between slopes and the interior, to energy pathways in the ocean, the suspension and settling of sediment, and to local biological productivity. It is thus necessary to improve our understanding and our ability to numerically predict the internal wave/boundary interaction problem and a multi-scale approach, with potentially transformative outcomes, will bridge the large internal wave scales to the bottom boundary layer (BBL). Realistic simulations of bottom mixing processes have not been possible so far because of the scale disparity between the incident wave field and the turbulence, the high Reynolds number of pre-existing boundary layers as well as those that form in the case of critical slope, and the high numerical dissipation used for stability in current ocean models. The scale-separation problem is handled here through a novel hierarchical use of analytical or numerical solutions for the large-scale problem coupled to a large eddy simulation (LES) of bottom mixing. To provide the large-scale field when analytical theories are not available, we will employ a new baroclinic model based on Adaptive Mesh Refinement (AMR). A realistic description of small-scale processes will be obtained here by employing LES with a sophisticated sub-grid model. This sub-grid model has a scale-similarity component to account for deviations from isotropic, inertial-range behavior, it is not unduly dissipative, it does not need tunable coefficients, and it has a near-wall component to account for the turbulence at the boundary layer roughness scale. The AMR and LES will be two-way coupled. The AMR (when needed) will drive the LES, while the latter will provide approximated boundary conditions for the former allowing communication of boundary mixing into the interior. The simulations will be analyzed for BBL properties, phasing of local mixing rates, lateral dispersal of BL mixed fluid, and characteristics of the nonlinearly scattered wave. Functional relationships to the properties of the incoming wave and to the slope angle will be extracted. Comparisons with laboratory and field data will be performed. Numerical large-scale models at the regional or global level are asked to provide increasingly precise evaluations of oceanic impact on climate change, depend on parameterizations for boundary fluxes. The AMR and LES simulations proposed here will allow a careful assessment of bottom turbulence dynamics which could then be used to develop an accurate representation of the bottom mixing component of large-scale models. Mixing along slopes affects several characteristics of the shelf-edge/slope area, which in turn control the exchange of nutrients and pollutants between the boundary and the open ocean. On a broader horizon, mixing at hot spots on shelf slopes is a critical ingredient in the energy balance of the internal wave field. Two PhD students will be trained across the fields of oceanography, engineering and computational science. Undergraduate students will be given the opportunity to participate in the research.