Generation Mechanisms of Nonlinear Internal Waves in Coastal Plain Estuaries

这项研究旨在极大地促进我们对河口混合过程的理解，并改善对河口水质和生物地球化学循环的预测。用于预测河口环境条件的数值模型往往难以再现层化作用，影响了我们准确预测各种生物地球化学过程的能力，包括底水缺氧。这可能与在跃层中波诱导混合缺乏适当的参数有关。非线性内波普遍存在于沿海层结海洋中，在产生湍流混合和输送重要的生物化学物质方面起着重要作用。然而，在分层河口，除了深峡口入口处的水流外，很少有人研究非线性内波。河口内的湍流和混合主要是由底部边界层湍流引起的，而对破碎内波的研究较少。该项目将重新分析实验数据，并使用数值模式来研究内波在河口混合中的作用。它将为一名博士生提供教育机会，他们将接受数字海洋建模和高级数据分析技术方面的培训。此外，本科生将在夏季参与这个项目。沿海平原河口的潮流主要沿着等深方向移动，被认为不利于内波的产生。另一方面，大多数沿海平原河口以深水道为特征，两侧是浅滩，在跨海峡方向上有陡峭的水深变化。此外，在许多河口还观察到了横向环流。假设当内弗劳德数大于1时，侧向环流与航道浅滩测深的相互作用会导致内背风波的产生。后者通过非线性陡化和频散演化为一系列非线性内波。并假设在强潮或强风条件下，侧向环流产生锋生和底层重力流，激发上游传播的非线性内波。这些假设基于切萨皮克湾最近对内背风波和内孤立波的观测，并将使用面向过程的真实模拟进行验证。之前收集的系泊数据将被重新分析，以更好地量化非线性内波和河口湍流混合之间的关系。切萨皮克湾的一个真实的3-D流体静力模型将被运行来进行后播模拟和解释观测数据。利用在理想河口航道上配置的静力和非静力模型，研究河口内波的产生、传播和转化。波浪产生机制将在大范围的河流水流、潮汐和风力强迫条件下进行探索。这些模型运行将得到数据分析的补充，以确定非线性内波和河口混合之间的联系。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This research aims at significantly advancing our understanding of estuarine mixing processes and improving the predictions of water-quality and biogeochemical cycling in estuaries. Numerical models used in predicting environmental conditions in estuaries often have difficulties in reproducing stratification affecting our ability to accurately predict various biogeochemical processes, including bottom water hypoxia. This may be related to the lack of proper parameterizations for wave-induced mixing in the pycnocline. Nonlinear internal waves are ubiquitous in the stratified coastal ocean and play an important role in generating turbulent mixing and transporting biochemically important materials. However, nonlinear internal waves have been rarely studied in stratified estuaries, except for flows over sills at the entrance to deep fjords. Turbulence and mixing in estuaries has been mainly attributed to bottom boundary layer turbulence whereas little attention has been paid to breaking internal waves. This project will re-analyze experimental data and use numerical models to examine the role of internal waves in estuarine mixing. It will provide educational opportunities for one Ph.D. graduate student who will be trained in numerical ocean modeling, and advanced data analysis techniques. In addition, undergraduate students will be involved in this project during the summer months.Tidal currents in coastal plain estuaries primarily move in the along isobath direction and are thought to be no-conducive to internal wave generation. On the other hand, most coastal plain estuaries feature a deep channel flanked by shallow shoals, with steep bathymetric changes in the cross-channel direction. Moreover, lateral circulation has been observed in many estuaries. It is hypothesized that when the internal Froude number exceeds 1, the interaction between the lateral circulation and channel-shoal bathymetry leads to the generation of an internal lee wave. The latter evolves into a train of nonlinear internal waves through nonlinear steepening and dispersion. It is also hypothesized that under strong tidal or wind forcing conditions the lateral circulation generates frontogenesis and a bottom gravity current which excites upstream propagating nonlinear internal waves. These hypotheses are based on the recent observations of internal lee waves and internal solitary waves in Chesapeake Bay and will be tested using process-oriented and realistic modeling. The previously collected mooring data will be re-analyzed to better quantify the relationship between nonlinear internal waves and estuarine turbulent mixing. A realistic 3-D hydrostatic model of Chesapeake Bay will be run to conduct hindcast simulations and interpret the observational data. The generation, propagation and transformation of internal waves in estuaries will be investigated using hydrostatic and non-hydrostatic models configured over an idealized estuarine channel. The wave generation mechanisms will be explored under a wide range of river flow, tidal and wind forcing conditions. These model runs will be complemented by data analysis to identify the connection between nonlinear internal waves and mixing in the estuary.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.