Collaborative Research: Entrainment in Dense Currents Over a Rough Bottom

概述：控制密集流夹带的动力学对海洋中密集水的形成、运动和分布至关重要;是温盐环流的基石。然而，这些海流的夹带和混合发生在如此小的尺度上，并且流动如此迅速，以至于在全球海洋环流和气候模型中，目前很难完全解析动力学，如果不是不可能的话。因此，夹带，往往是溢出本身没有解决，需要parameterized.Intellectual优点：现有的参数化夹带在密集的电流占主要是剪切诱导夹带在密集流和周围流体之间的界面。然而，在底部边界处由粗糙元件产生的湍流产生增强的阻力，是强烈的并且不能被忽略。在本项目中，假设对于高度远大于底部边界层厚度的密流，底部粗糙度产生的湍流涡流将在密流中发挥作用，但不会导致密流中周围沃茨的夹带（即不会导致水特性的变化）。相反，对于高度与底部边界层厚度相当或小于底部边界层厚度的密集水流，底部附近的湍流涡流应足够大，以夹带位于密集水流上方的环境水，并应显著影响密集水的性质。因此，在卷吸参数化中应包括底质粗糙度引起的卷吸效应，并确定底质粗糙度重要的参数范围，本项目将通过实验室和数值研究相结合的方法解决这一缺陷，重点研究大范围粗糙底质上的卷吸和密流动力学，其中形状粗糙度元素的形状（圆形、正方形和三角形横截面）、垂直范围、间距（稀疏与密集配置）和空间分布（规则与不规则）将变化。研究者将：（i）量化卷吸与密流高度与底边界层厚度之比之间的关系;（ii）确定粗糙度单元的形状、垂直范围、间距和空间分布对底边界层厚度的影响，在底边界层中，湍流涡流预计会发展; ㈢建立一个新的通用卷吸参数化方法，其中考虑到海底粗糙度。因此，当这些电流流过粗糙的海底水深时，对这些电流的动态有更好的理解，以及夹带和粗糙度参数之间关系的发展有可能改变这些流动中混合参数化的方式。在实践中，该项目的结果可能导致先进的夹带参数化，在气候模型中重要水团的水柱中更现实的位置，因此，这些模型的预测能力得到改善。德州大学圣安东尼奥分校一名研究生的论文。学生将获得涉及数值模型和实验室实验的合作研究经验。两个PI都深入参与教学和咨询，这个项目的结果将很快进入研究生教育。Cenedese也是WHOI地球物理流体动力学（GFD）夏季课程的教师。在此期间，预计一名或多名GFD研究员将在她的指导下从事相关项目。最后，研究人员计划吸引研究生和本科生客座学生在WHOI工作约4个月的项目，以支持拟议的研究。WHOI的实验工作还将利用地球物理流体动力学实验室，该实验室具有提供设施的悠久传统，并协助来自美国和世界各地的学生和科学家在物理海洋学，地质学和生物物理相互作用领域进行流体动力学实验。实验和相关数值模型的视频将在网络上提供，并附有适合课堂使用的相关解释。

Overview:The dynamics controlling the entrainment in dense currents are fundamental to the formation, movement, and distribution of the densest water in the ocean; a cornerstone of the thermohaline circulation. However, the entrainment and mixing in these currents occurs at such small scales, and the flows are so rapid that full resolution of the dynamics is presently very difficult, if not impossible, in global ocean circulation and climate models. Consequently, the entrainment, and often the overflows themselves are not resolved and need to be parameterized.Intellectual Merit :The existing parameterizations for entrainment in dense currents account primarily for the shear-induced entrainment at the interface between the dense flow and the ambient fluid. However, the turbulence generated by roughness elements at the bottom boundary, which produces an enhanced drag, is intense and cannot be ignored. In this project, it is hypothesized that for dense currents having a height much larger than the bottom boundary layer thickness, the turbulent eddies generated by the bottom roughness will play a role in homogenizing the dense current but will not contribute to entrainment of ambient waters within the dense current (i.e. will not contribute to changes in the water properties). Conversely, for dense currents having a height comparable to or smaller than the bottom boundary layer thickness, the turbulent eddies near the bottom should be large enough to entrain the ambient water lying above the dense current and should significantly influence the dense water properties. The effect of entrainment due to bottom roughness should therefore be included in the entrainment parameterizations, and the parameter regime in which bottom roughness is important be identified.This project will address this shortcoming with a combined laboratory and numerical study focused on entrainment and dense currents dynamics over a wide range of rough bottoms, in which the shape (circular, square, and triangular cross section), vertical extent, spacing (sparse vs. dense configuration), and spatial distribution (regular vs. irregular) of the roughness elements will be varied. The investigators will: (i) quantify the relationship between entrainment and the ratio of the dense current height to the bottom boundary layer thickness; (ii) determine the influence of the shape, vertical extent, spacing, and spatial distribution of the roughness elements on the thickness of the bottom boundary layer in which turbulent eddies are expected to develop; (iii) establish a new universal entrainment parameterization which takes into account the bottom roughness.Broader Impacts :Overflows and dense currents are important aspects of the deep ocean circulation. As such, improved understanding of the dynamics of these currents when they flow over a rough bottom bathymetry, and the development of relationships between the entrainment and roughness parameters has the potential to change the way mixing is parameterized in these flows. In practice, results from this project could lead to advanced entrainment parameterizations, a more realistic location in the water column of important water masses in climate models, and hence, an improved prognostic power of these models.This project will support the Ph.D. thesis work of a graduate student at UT San Antonio. The student will gain experience in collaborative research involving numerical models and laboratory experiments. Both PIs are deeply involved in teaching and advising and the results from this project will quickly find their way into graduate education. Cenedese is also on the faculty of the Geophysical Fluid Dynamics (GFD) Summer Program at WHOI. Over the course of this grant, it is expected that one or more GFD fellows will work on related projects under her guidance. Finally, the investigators plan to attract graduate and undergraduate guest students to work at WHOI on ~ 4 month projects in support of the proposed study. The experimental work at WHOI will also utilize the Geophysical Fluid Dynamics Laboratory, which has a long tradition of providing facilities, and assisting students and scientists from around the US and world to conduct fluid dynamics experiments in the areas of physical oceanography, geology, and bio-physical interactions. Videos of experiments and related numerical models will be presented on the web with associated explanations suitable for classroom use.