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What can fine-scale stratification structure in thermal microscale profiles of lakes tell us about their turbulence history?

湖泊热微尺度剖面中的精细分层结构可以告诉我们什么关于它们的湍流历史？

基本信息

批准号：
NE/G010498/1
负责人：
Andrew Folkard
金额：
$ 6.47万
依托单位：
Lancaster University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FG010498%2F1
关键词：
What fine scale stratification structure

项目摘要

Turbulent mixing in density-stratified fluids is of central importance to the cycling of heat, and key biogeochemical elements from global to micro-scales, and thus to the climate and ecosystems of our planet. Despite this, a recent review by three world-leaders in the field of environmental turbulence [Ivey et al 2008] emphasises that our sampling of this phenomenon 'does not begin to approach' a reliable representation of its magnitude or spatio-temporal structure. As a result, there remain very large, untested areas of our understanding of problems ranging from the global-scale cycling of heat in the oceans, to the patterns of nutrient cycling in small lakes. This problem is so persistent because of the highly variable nature of turbulence, and the fact that current instrumentation can only measure it at a very limited number of points for any significant time, or for a short time (a quasi-instantaneous snapshot) at any significant number of points. The aim of this project is to investigate whether existing turbulence profile data can provide us with a detailed and reliable history of turbulent activity and thus allow us to expand the extent to which we can sample it. The approach focuses on the apparently ubiquitous (and hitherto not analysed) presence of small-scale layering that is revealed in micro-scale temperature profiles of thermally-stratified water bodies. While some (e.g. MacIntyre, pers. comm.) argue that they are caused by internal wave motions which locally squash and stretch the isotherms, thus distorting vertical temperature profiles, there is also the strong possibility that at least some are the result of local turbulent mixing. The current approach to analysing microstructure profiles for turbulence data considers only the evidence of active turbulence within the profile. It either identifies statistically stationary segments, from which it determines measures of vertical turbulent diffusivity, or it identifies turbulent patches. This project will compare the turbulence distributions identified through those methods with the fine-scale stratification structure. From this, relationships will be identified between measures of turbulent overturn characteristics and the underlying stratification, and distinctions made between regions of fine-scale stratification structure which are actively turbulent and those which are not. We will test the hypothesis that we can use the former to develop stratification-turbulence relationships that would allow use of the fine-scale stratification data as a proxy for turbulent activity, and then apply that relationship to the latter to quantify the historical turbulent activity evidenced in these footprints of now-decayed turbulence. We hypothesise further that this would, at least, give us an upper limit on the historical turbulence intensity (i.e. it would assume that all the fine-scale stratification structure was caused by now-decayed turbulence), and would provide pump-priming for further investigation of this phenomenon that would (a) investigate ways of distinguishing turbulence footprints from internal wave footprints; and (b) compare this physical fine scale structure with that in chemical and biological parameters and allow us to infer its biogeochemical and ecological relevance. The project addresses an issue - turbulent mixing - that is central to biogeochemical and thermal cycling in lakes, and thus efforts to enhance the quality of lakes and research into the role of lakes in global nutrient cycles and climate change.

密度分层流体中的湍流混合对于热循环、从全球到微观尺度的关键生物地球化学元素以及我们星球的气候和生态系统都至关重要。尽管如此，环境动荡领域的三位世界领袖最近发表的一篇综述[Ivey et al . 2008]强调，我们对这一现象的抽样“尚未开始接近”其规模或时空结构的可靠表示。因此，从全球范围的海洋热循环到小湖泊的营养循环模式，我们对问题的理解仍然有很大的、未经测试的领域。这个问题之所以持续存在，是因为湍流具有高度可变的性质，而且目前的仪器只能在任何重要时间内测量非常有限的点，或者在任何重要时间点上测量很短的时间（准瞬时快照）。该项目的目的是调查现有的湍流剖面数据是否可以为我们提供湍流活动的详细和可靠的历史，从而使我们能够扩大我们可以采样的程度。该方法侧重于在热分层水体的微尺度温度剖面中揭示的明显普遍存在的（迄今尚未分析的）小尺度分层。而有些人（如麦金太尔）认为。Comm .)认为它们是由局部挤压和拉伸等温线的内部波运动引起的，从而扭曲了垂直温度分布，至少有一些很可能是局部湍流混合的结果。目前分析湍流数据的微观结构剖面的方法只考虑了剖面内活跃湍流的证据。它要么识别统计上稳定的片段，从中确定垂直湍流扩散系数的测量，要么识别湍流斑块。本项目将通过这些方法确定的湍流分布与精细尺度的分层结构进行比较。由此，将确定湍流翻转特征测量值与下伏层理之间的关系，并区分精细尺度层理结构区域的活跃湍流和非活跃湍流。我们将测试假设，我们可以使用前者来发展分层-湍流关系，这将允许使用精细尺度分层数据作为湍流活动的代理，然后将该关系应用于后者，以量化这些现已衰减的湍流足迹所证明的历史湍流活动。我们进一步假设，这将至少给我们一个历史湍流强度的上限（即，它将假设所有的精细尺度分层结构都是由现在衰减的湍流引起的），并将为进一步研究这一现象提供泵启动，这将(a)研究区分湍流足迹和内波足迹的方法；(b)将这种物理精细尺度结构与化学和生物参数进行比较，从而推断其生物地球化学和生态相关性。该项目解决的问题是湍流混合，这是湖泊生物地球化学和热循环的核心问题，因此也是提高湖泊质量和研究湖泊在全球营养循环和气候变化中的作用的努力的核心。