Mixing in upper layer of a lake during heating cycle


LITTLE is yet known about the structure of the upper ‘mixing’ layer of the ocean below the zone of direct surface wave influence. We report some measurements of the temperature field in the mixing layer of a freshwater lake during the heating cycle which demonstrate some of the processes which may be important. The temperature field is found to be isotropic at scales less than about 60 cm, but the effect of stratification becomes important at larger scales, reducing the vertical correlations in comparison with the horizontal. The spatial evolution of structures is dominated by the shear, as appropriate in the low Richardson number region. Examination of the stability of the mean velocity and density profiles shows that Kelvin–Helmholtz instability may be expected, leading to billows with wavelengths of about 60 m, and corresponding to fluctuation scales observed in the temperature records. Similar fluctuations have been seen in the nocturnal atmospheric boundary layer.

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THORPE, S., HALL, A. Mixing in upper layer of a lake during heating cycle. Nature 265, 719–722 (1977). https://doi.org/10.1038/265719a0

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