Laboratory and Numerical Experiments on Ocean-scale Turbulent Stratified Mixing

该项目将利用实验室实验和数值模拟技术，查明海洋表面的沃茨通过一种称为翻转的过程与较深的较重沃茨混合的机制。世界各大洋中的水排列成密度随深度增加的近乎水平的层。由于密度的差异，材料在层之间的转移（即，密度大的水上升，密度小的水下沉）需要外部能源，整个过程称为混合。人们对理解分层水体中的混合现象有相当大的兴趣。这一过程是缓慢的;据估计，它需要大约1,000年才能完全推翻海洋。然而，由于深海储存了大量的热量和二氧化碳，翻转过程非常重要，因为它在千年甚至更长的时间尺度上调节地球上的气候，因此具有巨大的社会利益。分层混合需要能量来源（搅拌），本项目的目的是表征该过程的效率。要理解效率的概念，可以用汽车来类比。在发动机中燃烧的气体释放出一定量的能量，但只有一小部分用于驱动传动系统。其余的作为热量散失。同样地，只有一小部分由搅拌机构提供的能量用于克服重力做功，其余的作为摩擦热耗散。例如，使用声学传感器和快速温度传感器以越来越高的分辨率对海洋混合进行的直接观测显示出剪切驱动湍流中令人惊讶的结构，这表明海洋尺度上的混合可能与小规模实验室测量所显示的不同。该项目通过在与海洋流动有关的尺度上仔细控制的实验室实验中研究湍流混合和使用数值模拟来解决这些问题。实验部分是在CRAWJoint流体实验室的水槽中进行的，这是一个36米长的设施，可以产生具有高浮力雷诺数的分层剪切流。这确保了柯尔莫哥洛夫和奥兹米多夫尺度之间的明确分离。实验的目的是建立的行为的通量理查森数（效率的措施），在高浮力雷诺数和有限梯度理查森数的限制。在实验室中获得的数据用于通过数据同化训练一套日益复杂的数值模型，最终目标是创建可用于诊断海洋数据效率的工具，并改进全球模型中分层混合的参数化。

This project will use laboratory experiments and numerical modelling techniques to identify the mechanism by which waters in the surface of the ocean are mixed with heavier deeper waters through a process that is called overturning. Water in the world oceans is arranged in nearly-horizontal layers of density increasing with depth. Because of the difference in density, transfer of material between layers (i.e., lifting of denser water and sinking of lighter water) requires an external source of energy and the whole process is called mixing. There is considerable interest in understanding mixing in stratified bodies of water. This process is slow; it is estimated that it takes about 1,000 years to completely overturn the ocean. Yet, since the deep ocean holds large stores of heat and carbon dioxide, the overturning process is important as it regulates climate on the planet over millennial and longer time scales, and thus of great societal interest.Stratified mixing requires a source of energy (stirring), and the aim of this project is to characterize the efficiency of the process. To understand the concept of efficiency, it is convenient to think by analogy to an automobile. The gas burned in the engine releases a certain amount of energy, but only a fraction of it goes into moving the drivetrain. The rest is lost as heat. Likewise, only a fraction of the energy provided by the stirring mechanism goes into doing work against gravity, the rest being dissipated as frictional heat. Direct observations of ocean mixing with increasingly high resolution using, for example, acoustic sensors and fast temperature sensors, have shown surprising structures in shear-driven turbulence that suggest mixing at ocean scales may behave differently than small-scale laboratory measurements have suggested. This project addresses these issues through the study of turbulent mixing in carefully controlled laboratory experiments at scales relevant to oceanic flows and the use of numerical simulations. The experimental component is conducted in the UNC Joint Fluid Lab tank, a 36m long facility in which stratified shear flows with high buoyancy Reynolds number can be generated. This ensures a clear separation between the Kolmogorov and Ozmidov scales. The experiments are designed to establish the behavior of the flux Richardson number (a measure of efficiency), in the limit of high buoyancy Reynolds number and finite gradient Richardson number. The data acquired in the lab are used to train, via data-assimilation, a suite of increasingly complex numerical models, with the final goal of creating tools that can be used to diagnose efficiency from ocean data and to improve the parameterization of stratified mixing in global models.