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Spatial variability of vertical eddy diffusivity in small lakes

小湖泊垂直涡扩散率的空间变异性

基本信息

批准号：
NE/F00995X/1
负责人：
Andrew Folkard
金额：
$ 5.59万
依托单位：
Lancaster University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2008
资助国家：
英国
起止时间：
2008 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FF00995X%2F1
关键词：
Spatial variability vertical eddy diffusivity

项目摘要

Lakes are vitally important elements of the global system. A keynote lecture at a recent, major international lake science conference presented compelling evidence that, since processes central to global nutrient cycles (such as carbon burial) are many times more efficient in lakes than they are on land, lakes are in some respects more important for these cycles than the land surface. They are also unique and often vulnerable habitats that support a very high species diversity and thus contribute greatly to the ecological health of the environment. In order for lakes to function and deliver these key contributions to the well-being of the natural environment, chemical and biological material must be moved efficiently around their water bodies. In particular, nutrient-rich material must be cycled up from the lake bed (to where it sinks if otherwise undisturbed) to the light, near surface waters where primary producing phytoplankton need to reside to be able to photosynthesise and bring the nutrients into the food chain. This vertical transport is achieved overridingly through vertical turbulent eddy diffusion: the vertical movement of dissolved and particulate material by turbulent eddies. Thus, in order to understand, and therefore be able to manage lakes to sustain their good health, it is imperative to understand and be able to compute accurately their vertical turbulent diffusivity. There are several ways of doing this, and our previous research has compared the reliability of these in the specific environment of small lakes, a category which includes the large majority of the world's, and the UK's, lakes. The results from this work suggest that turbulent eddy diffusivity is highly spatially variable across lakes, and that this variability may be largely due to the way in which wind speed at the water's surface (which is a primary cause of the turbulent diffusion) varies with fetch (the distance over which the wind has blown over the water surface at any given point). As a result, wind speeds measured in mid-lake locations, which are typically where lake meteorology is sampled, do not appear to be appropriate to use in calculating turbulent vertical mixing in lakes. This project will investigate this theory more fully, and will significantly enhance our ability to predict this key variable in small lakes. It will do so by measuring diffusivity using a temperature micro-profiler, which measures vertical water temperature profiles at specific locations with a resolution of approximately 1mm, from which profiles of diffusivity can be calculated using established methods. By measuring multiple profiles of diffusivity at different locations within two different lakes under a range of meteorological and ambient water stratification conditions, we will build a data set that we will then be able to anlayse to determine how wind fetch variations affect turbulent eddy diffusivity in lakes under a wide variety of conditions. Thus we will develop a robust ability to determine the correct way in which to use wind speed measurements to calculate vertical turbulent eddy diffusivity in small lakes.

湖泊是全球系统中极其重要的组成部分。在最近一次重要的国际湖泊科学会议上的一次主旨演讲提供了令人信服的证据，表明由于全球营养循环(如碳埋藏)的核心过程在湖泊中的效率比在陆地上高出许多倍，在某些方面，湖泊在这些循环中比陆地表面更重要。它们也是独特的、往往是脆弱的栖息地，支持着非常高的物种多样性，因此对环境的生态健康做出了巨大贡献。为了让湖泊发挥作用，并为自然环境的福祉做出这些关键贡献，化学和生物材料必须在其水体中有效地移动。特别是，营养丰富的物质必须从湖床(否则不受干扰就会下沉)循环到光线下，靠近表层水域，初级浮游植物需要在那里栖息，才能进行光合作用，并将营养物质带入食物链。这种垂直输运是通过垂直湍流涡流扩散实现的：通过湍流涡流对溶解和颗粒物质的垂直运动。因此，为了了解并因此能够管理湖泊以维持它们的良好健康，必须了解并能够准确地计算它们的垂直湍流扩散率。有几种方法可以做到这一点，我们之前的研究已经在小湖泊的特定环境中比较了这些方法的可靠性，这一类别包括世界上大多数湖泊和英国的湖泊。这项工作的结果表明，湖泊间的湍涡扩散率在空间上是高度可变的，这种变异性很大程度上是由于水面的风速(这是湍流扩散的一个主要原因)随FETCH(风在任何给定点吹过水面的距离)而变化的方式。因此，在湖中央位置测量的风速似乎不适合用于计算湖泊中的湍流垂直混合，湖中位置通常是湖泊气象采样的地方。这个项目将更全面地研究这一理论，并将显著增强我们在小湖泊中预测这一关键变量的能力。为此，它将使用温度微剖面仪测量扩散率，该剖面仪测量特定位置的垂直水温分布，分辨率约为1毫米，可使用现有方法计算扩散率分布。通过在一系列气象和环境水分层条件下测量两个不同湖泊内不同位置的多个扩散率剖面，我们将建立一个数据集，然后我们将能够分析该数据集，以确定在各种条件下风的取值变化如何影响湖泊中的湍流涡流扩散率。因此，我们将开发一种强大的能力来确定使用风速测量来计算小湖泊中垂直涡旋扩散率的正确方法。