Collaborative Research: Internal Swash zones and boundary-interior exchange: High-accuracy modeling and field observations

这个项目将研究如何强烈的非线性内波（即，在具有不同密度的两种流体的界面处产生的波）在床附近和在两种流体的界面处引起混合。将在一个南北走向的指状湖（纽约州卡尤加湖）进行现场观测，这是一个天然实验室，其条件类似于夏季在斜坡附近的外大陆架边缘发现的条件（即，温暖的表层覆盖在较冷的水层上）。对实验数据的分析与复杂的数值实验相结合，将提供资料，填补实验室工作与最新海洋学研究之间的差距。这项工作可能会提高参数化的边界通量在海洋中，这是重要的侧向分散的浮力，热量和潜在的有机碳离岸生产架系统。这项工作可能会促进跨学科的活动，因为它可能会改善物理海洋学和物理湖沼学之间的沟通。此外，研究结果也可能与湖泊和沿海地区的水质问题有关。卡尤加湖一直受到夏末藻类水华的困扰，同时为纽约州伊萨卡市和康奈尔大学提供饮用水。虽然这一问题不是研究的直接主题，但收集的数据将有助于解决这一具有重要社会意义的问题。该项目将支持两名研究生和一名本科生的培训，并提供研究材料，这些材料将被整合到主要研究人员的课堂活动中。该项目将建模和观测相结合，以研究非线性内波（NLIWs）如何影响近边界混合和边界内部交换事件。其目标是：1）在入射内波非线性的参数空间中探索NLIW不稳定性的机制，并评估波阵面中的底部剪切应力、对流/剪切不稳定性或波浪足迹中的底部边界层分离的作用; 2）量化和对比每种不稳定性类型对横扇通量产生和边界-内部交换的贡献（入侵生成）作为入射NLIW能量的函数; 3）确定平流冲流区的时间变化和由此产生的周期性入射NLIW和潜在的波浪-波浪相互作用在建立近床水的情景湍流、混合、水平输送和重新分层中的作用。数值模型是基于高精度谱元方法，使高分辨率的研究内波破碎的变化水深。纽约卡尤加湖的南坡和陆架为项目实地研究提供了一个特殊的天然实验室，作为一个简单的海洋模拟系统。这个湖模拟了许多夏季大陆架和斜坡上的情况，温暖的表层与大陆架相交，覆盖着一个分层层，向下延伸到一个更陡的水深斜坡上。低频假潮使密度跃层在陆架/斜坡上来回平流，并在边界附近破裂，混合分层流体并有助于将边界层流体驱逐到盆地的分层内部。对基础物理学的更好理解将与广泛的领域相关，并有助于相关过程的更好参数化。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project will examine how strongly non-linear internal waves (i.e., waves developed at the interface of two fluids with different density) induce mixing near the bed and at the interface of the two fluids. In-situ observations will be carried out in a north-south oriented finger-lake (Lake Cayuga, NY), a natural laboratory with conditions analogous to those found at the edge of the outer continental shelf near the slope during summertime (i.e., warm surface layer overlying a colder layer of water). The analysis of the experimental data combined with sophisticated numerical experiments will provide information that will fill the gap between laboratory work and the most up-to-date oceanographic studies. This work may improve parameterization of boundary fluxes in the ocean, which are important to lateral dispersion of buoyancy, heat, and potentially of organic carbon offshore of productive shelf systems. The work may advance interdisciplinary activities as it may improve communication between physical oceanography and physical limnology. In addition, results may also be pertinent to water quality issues in lakes and coastal areas. Lake Cayuga has been plagued by late-summer algal blooms, while supplying drinking water for the city of Ithaca NY, and for Cornell University. Although this problem is not the direct subject of the study, the data collected will be useful for addressing this societally important issue. The project will support the training of two graduate and one undergraduate student and provide research material that will be integrated in the classroom activities of the principal investigators.The project combines modeling and observations to investigate how nonlinear internal waves (NLIWs) affect near-boundary mixing and boundary-interior exchange events. The objectives are to: 1) explore, in the parameter space of incident internal wave nonlinearity, the mechanisms of NLIW instability, and assess the roles of bottom shear stress, convective/shear instability in the wave front, or bottom boundary layer separation in the wave footprint; 2) quantify and contrast the contribution of each instability type to diapycnal flux generation and boundary-interior exchange (intrusion generation) as a function of incident NLIW energy; 3) determine the role of temporal variability of the advective swash zone and the resulting cyclically incident NLIWs and potential wave-wave interactions, in establishing episodic turbulence, mixing, horizontal transport, and re-stratification of near-bed water. The numerical model is based on high-accuracy spectral element methods, to enable the high- resolution study of internal wave breaking over variable bathymetry. The southern slope and shelf of Lake Cayuga, New York, provides an exceptional natural laboratory for the project field studies, serving as a simpler analog system for the ocean. This lake mimics conditions seen on many summer continental shelves and slopes with a warm surface layer intersecting the shelf, overlying a stratified layer extending down over a steeper bathymetric slope. A low-frequency seiche advects the pycnocline back and forth across the shelf/slope, and hosts higher frequency NLIWs which break near boundaries, mixing the stratified fluid and contributing to expulsion of boundary layer fluid into a basin’s stratified interior. Better understanding of the basic physics will be relevant to a wide range of fields and contribute to better parameterization of the relevant processes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.