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UBC Theses and Dissertations

A laboratory and mathematical study of the 'thermal bar' Elliott, Gillian Hope

Abstract

The 'migrating thermal bar' phenomenon, known to occur in certain large, dimictic, freshwater lakes, has been studied in laboratory and mathematical models. The temperature fields observed in the laboratory agreed with those observed in Lake Ontario and a linear physical model for the speed of the 'thermal bar', based on negligible horizontal advection and diffusion of heat, gave reasonable values for both the laboratory model and Lake Ontario. Observations were also made of the associated velocity field. On the basis of this laboratory model, which suggests that horizontal advection and diffusion of heat were not of primary importance, mathematical models were developed. First the temperature field was calculated from the one-dimensional heat diffusion equation. Then the velocity field was calculated assuming that the flow was driven by buoyancy forces and balanced by viscous forces. On the basis of the similitude between the temperature fields found in my models and those observed in the lakes, it seems possible that the velocity field of the models also provides a good approximation to the circulation associated with the bar in lakes. There are no direct measurements of the velocities associated with the bar in lakes and they will be difficult to obtain as such velocities are expected, in Lake Ontario, to be only of the order of 1 cm sec⁻¹.

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