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

该项目将研究强烈的非线性内波（即在两种不同密度流体的界面处形成的波）如何在床附近和两种流体的界面处诱导混合。现场观测将在一个南北向的指状湖（纽约州的卡尤加湖）进行，这是一个自然实验室，其条件类似于夏季在靠近斜坡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）。对实验数据的分析结合复杂的数值实验将提供信息，填补实验室工作与最新海洋学研究之间的空白。这项工作可能会改善海洋边界通量的参数化，这对浮力、热量和有机碳在生产陆架系统近海的横向分散是重要的。这项工作可以促进跨学科活动，因为它可以改善物理海洋学和物理湖沼学之间的交流。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华的困扰，同时为纽约伊萨卡市和康奈尔大学提供饮用水。虽然这个问题不是研究的直接主题，但所收集的数据将有助于解决这一重要的社会问题。该项目将支持培训两名研究生和一名本科生，并提供将纳入主要研究人员课堂活动的研究材料。该项目将建模和观测相结合，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。研究目标是：1)在入射内波非线性参数空间中探索NLIW不稳定机制，评估波前底部剪切应力、对流/剪切不稳定或波足迹底部边界层分离在NLIW不稳定中的作用；2)量化和对比每一种不稳定类型作为入射NLIW能量的函数对底波通量产生和边界-内部交换（入侵产生）的贡献；3)确定平流冲刷带的时间变率以及由此产生的周期性NLIWs和潜在波-波相互作用在建立幕式湍流、混合、水平输送和近床水再分层中的作用。该数值模型是基于高精度谱元方法，以实现高分辨率研究变水深的内波破碎。纽约卡尤加湖的南部斜坡和陆架为项目的实地研究提供了一个特殊的自然实验室，作为一个简单的海洋模拟系统。这个湖模拟了在许多夏季大陆架和斜坡上看到的情况，温暖的表层与大陆架相交，覆盖在一个分层层上，向下延伸到一个更陡峭的等深线斜坡上。低频淤积沿斜斜在陆架/斜坡上来回流动，并形成高频NLIWs，这些NLIWs在边界附近断裂，混合了分层流体，并将边界层流体驱逐到盆地的分层内部。对基础物理的更好理解将与广泛的领域相关，并有助于更好地参数化相关过程。该奖项反映了美国国家科学基金会的法定使命，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.