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

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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.