Collaborative Research: Entrainment in Dense Currents Over a Rough Bottom

合作研究：粗糙底部浓密水流的夹带

• 批准号: 1333174
• 负责人: Claudia Cenedese
• 金额: $ 47.74万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-15 至 2018-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1333174&HistoricalAwards=false
• 关键词: Collaborative; Research; Entrainment; Dense; Currents

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.

概述：控制密集洋流中夹带的动力学是海洋中密度最大的水的形成、运动和分布的基础；是温盐环流的基石。然而，这些洋流中的夹带和混合发生在如此小的尺度上，并且流动如此之快，以至于目前在全球海洋环流和气候模型中，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，夹带和溢流本身并没有得到解决，需要进行参数化。智能优点：现有的密流中夹带的参数化主要考虑了在密流和周围流体之间的界面处剪切诱导的夹带。然而，由底部边界处的粗糙元素产生的湍流会产生增强的阻力，其强度是不容忽视的。在这个项目中，假设对于高度远大于底部边界层厚度的致密流，由底部粗糙度产生的湍涡将起到均化致密流的作用，但不会有助于将周围水域卷吸到致密流中(即不会导致水性质的变化)。相反，对于高度等于或小于底部边界层厚度的致密流，底部附近的湍涡应该足够大，以卷吸位于致密流上方的环境水，并应显著影响致密水的性质。这一项目将通过实验室和数值研究相结合的方法来解决这一缺点，该研究集中在大范围粗糙海底的卷吸和密流动力学，其中粗糙元素的形状(圆形、正方形和三角形横截面)、垂直范围、间距(稀疏和密集形状)和空间分布(规则和不规则)将发生变化。研究人员将：(1)量化夹卷与密流高度与底部边界层厚度的比率之间的关系；(2)确定粗糙元素的形状、垂直范围、间距和空间分布对预计将产生湍流的底部边界层厚度的影响；(3)建立一个新的考虑底部粗糙度的新的通用夹带参数化。因此，更好地了解这些洋流在粗糙的海底水深测量中流动时的动力学，以及卷吸作用和粗糙度参数之间关系的发展，有可能改变在这些流动中混合的参数化方式。在实践中，这个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，从而改善这些模型的预测能力。该项目将支持德克萨斯大学圣安东尼奥分校一名研究生的博士论文工作。学生将获得合作研究的经验，包括数值模型和实验室实验。这两家私人投资机构都深入参与了教学和咨询工作，这个项目的结果将很快进入研究生教育。Cenedese也是世界卫生组织地球物理流体动力学(GFD)夏季项目的教员。在这笔赠款期间，预计将有一名或多名GFD研究员在她的指导下从事相关项目的工作。最后，研究人员计划吸引研究生和本科生到世界卫生组织工作，进行为期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。实验的视频和相关的数值模型将在网络上展示，并提供适合课堂使用的相关解释。