Turbulence measurements in the water-side surface boundary layer of a boreal lake

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Title: Turbulence measurements in the water-side surface boundary layer of a boreal lake
Author: Vähä, Aki
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
URI: http://urn.fi/URN:NBN:fi-fe201804208686
Thesis level: master's thesis
Discipline: Geophysics
Abstract: Lakes in the boreal region hold a significant importance in the global carbon cycle. They transport and store carbon and exchange it with the atmosphere. As turbulent transport is the most important process in transporting substances in water and air, special weight has been laid on studying turbulent processes in lakes. This work concentrates on turbulence in the surface boundary layer of Lake Kuivajärvi in Western Finland. A 16-day measurement campaign was carried out in Kuivajärvi in September 2014. An acoustic Doppler velocimeter (ADV) was used for high-frequency velocity measurements. The meteorological forcing had two distinct regimes during the campaign: a relatively calm and warm period during the first 13 days and a cold and windy period during the last three days. The two regimes were visible in the measured velocity as well as in the calculated turbulence parameters. The friction velocity in water was estimated to be 3*10^-4...2*10^-2 m/s during low winds and 2*10^-3...5*10^-2 m/s during high winds. In the low-wind regime, the friction velocity in water was generally smaller than the scaled friction velocity in air. In the high-wind regime, the situation was opposite. The common practice of estimating the water-side friction velocity from above-surface measurements isn't justified in all conditions. The viscous dissipation rate was calculated using the inertial subrange method and the neutral scaling. Dissipation rate estimates were from ~10^-10 to 10^-4 W/kg during the low-wind regime. During the high-wind regime, the estimates ranged from ~10^-7 to 10^-3 W/kg. It is likely that the highest dissipation rate estimates were erroneous as such high values have not been reported elsewhere in lakes. A simplified turbulent kinetic energy equation with dissipation rate, shear production and buoyancy production was tested. There was an imbalance of turbulence production and dissipation that was also related to the two meteorological regimes. Whether the equation should also include turbulent transport terms is an open question. The most important sources for errors were noise in the velocity time series and the effect of waves. It was shown that noise removal is an essential part of the ADV data analysis, however, noise-removal methods and methods for calculating the dissipation rate should be developed further. The installment of the ADV instrument should also be improved.

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