Laboratory and Numerical Experiments on Ocean-scale Turbulent Stratified Mixing

该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种称为倾覆的过程与更重的更深的水混合的机制。世界海洋中的水几乎是水平排列的，密度随着深度的增加而增加。由于密度的不同，物质在层与层之间的转移(即，提升较稠密的水和下沉较轻的水)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有相当大的兴趣。这个过程是缓慢的；据估计，大约需要1000年才能完全颠覆海洋。然而，由于深海储存了大量的热量和二氧化碳，倾覆过程非常重要，因为它在千年和更长的时间尺度上调节着地球上的气候，因此具有巨大的社会利益。分层混合需要一种能源(搅拌)，该项目的目的是表征这一过程的效率。要理解效率的概念，将其类比为汽车是很方便的。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动力总成。其余的则以热量的形式消失。同样，搅拌机构提供的能量中只有一小部分用于做反重力的功，其余的以摩擦热的形式消散。例如，使用声学传感器和快速温度传感器对分辨率越来越高的海洋混合进行的直接观测显示，剪切驱动的湍流中存在令人惊讶的结构，这表明海洋尺度上的混合行为可能与小规模实验室测量所表明的不同。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。实验部件是在北卡罗来纳大学联合流体实验室的水箱中进行的，这是一个36米长的装置，可以在其中产生高浮力雷诺数的分层剪切流动。这确保了科尔莫戈洛夫音阶和厄兹米多夫音阶之间的明显分离。实验的目的是确定在高浮力雷诺数和有限梯度理查森数的极限下，通量理查森数(衡量效率的一个指标)的行为。在实验室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式，最终目标是创造可用于从海洋数据诊断效率的工具，并改进全球模式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.