Laboratory and Numerical Experiments on Ocean-scale Turbulent Stratified Mixing

这个项目将使用实验室实验和数值模拟技术来确定海洋表面的水通过一个称为倾覆的过程与较重的深层水混合的机制。世界海洋中的水几乎呈水平排列，密度随深度增加而增加。由于密度的不同，物质在层之间的转移（即较密的水的上升和较轻的水的下沉）需要一个外部的能量来源，整个过程被称为混合。人们对了解分层水体中的混合现象非常感兴趣。这个过程是缓慢的；据估计，完全翻覆海洋需要大约1000年的时间。然而，由于深海储存着大量的热量和二氧化碳，这一翻转过程很重要，因为它调节着地球上数千年乃至更长的时间尺度上的气候，因此具有重大的社会利益。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。为了理解效率的概念，用汽车来类比是很方便的。在发动机中燃烧的气体释放出一定量的能量，但其中只有一小部分用于驱动传动系统。其余的以热量的形式散失。同样，由搅拌机构提供的能量中，只有一小部分用于对重力做功，其余部分作为摩擦热散去。使用声波传感器和快速温度传感器等越来越高分辨率的海洋混合直接观测显示，剪切驱动湍流的惊人结构表明，海洋尺度上的混合可能表现得与小规模实验室测量结果不同。该项目通过在与海洋流动相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。实验部件在UNC联合流体实验室槽中进行，该槽长36m，可产生高浮力雷诺数的分层剪切流。这确保了Kolmogorov和Ozmidov尺度之间的明确分离。实验旨在建立在高浮力雷诺数和有限梯度理查德森数的极限下通量理查德森数（效率的度量）的行为。通过数据同化，实验室获得的数据用于训练一套日益复杂的数值模型，其最终目标是创建可用于从海洋数据诊断效率的工具，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.