Collaborative Research: Entrainment in Dense Currents Over a Rough Bottom

概述：控制密流夹带的动力学是海洋中最密水的形成、运动和分布的基础；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，而且流动速度如此之快，以至于目前在全球海洋环流和气候模式中，要完全解决动力学问题是非常困难的，如果不是不可能的话。因此，夹带和通常的溢出本身没有得到解决，需要参数化。智力优势：致密流中夹带的现有参数化主要考虑了在致密流和周围流体之间的界面上剪切引起的夹带。然而，由底部边界的粗糙度元素产生的湍流产生了增强的阻力，是强烈的，不可忽视的。在本项目中，我们假设对于高度远大于底边界层厚度的密集流，由底部粗糙度产生的湍流涡流将起到均匀密集流的作用，但不会对密集流内的环境水产生夹带作用（即不会对水的性质产生改变）。相反，对于高度与底边界层厚度相当或小于底边界层厚度的稠密流，底部附近的湍流涡流应足够大，以夹带位于稠密流上方的环境水，并应显著影响稠密水的性质。因此，由于底部粗糙度引起的夹带的影响应包括在夹带参数化中，并且应确定底部粗糙度重要的参数体系。本项目将通过结合实验室和数值研究来解决这一缺点，重点是在大范围粗糙底部上的夹带和密集电流动力学，其中粗糙度元素的形状（圆形，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）将会变化。研究人员将：(i)量化夹带与密集电流高度与底部边界层厚度之比之间的关系；（ii）确定粗糙度要素的形状、垂直范围、间距和空间分布对预计会形成湍流涡流的底边界层厚度的影响；（iii）建立一个考虑底部粗糙度的新的通用夹带参数化。更广泛的影响：溢出和密集的洋流是深海环流的重要方面。因此，当这些水流流过粗糙的海底水深时，对其动力学的更好理解，以及对携流和粗糙度参数之间关系的发展，有可能改变这些水流中混合参数化的方式。在实践中，该项目的结果可能导致更先进的携带参数化，使气候模式中重要水团在水柱中的位置更加真实，从而提高这些模式的预测能力。该项目将支持德州大学圣安东尼奥分校一名研究生的博士论文工作。学生将获得涉及数值模型和实验室实验的合作研究经验。两位pi都深入参与教学和咨询，该项目的成果将很快应用于研究生教育。Cenedese也是WHOI地球物理流体动力学（GFD）暑期项目的教员。在这项资助的过程中，预计将有一名或多名GFD研究员在她的指导下从事相关的项目。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具有悠久传统的地球物理流体动力学实验室，该实验室提供设施，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。实验视频和相关的数值模型将在网上提供，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.