Air-water exchange and energy flux paths in small lakes
Final Report Abstract
Wind-induced water motions play a vital role in the ecology and biogeochemistry of lentic aquatic ecosystems, such as lakes and reservoirs. Wind stress acting at the water surface generates a turbulent mixing layer, surface waves, large-scale flow structures, as well as internal waves, which can convey energy to greater depths. The surface momentum transfer and the corresponding intensity of boundary-layer turbulence also affects the exchange of heat and dissolved gases between the lake surface and the atmosphere, and the rate of evaporation. The processes that control air-water exchange have mainly been studied in the ocean and at high wind speeds. Not much is known about the rate of momentum transfer and its interrelation to other transfer coefficients in small lakes, where the wind speed and fetch length are typically small. In this project, we compiled recently conducted atmospheric eddy-covariance (EC) measurements of momentum, heat, water vapor over more than 30 lakes and reservoirs. This unique data set was used to analyze the dependence of the bulk transfer coefficients on wind speed. We found that the drag coefficient, which describes momentum transfer, increases strongly at low wind. In addition, it showed contrasting dependence on lake size at low and high wind speed. In three selected water bodies with ongoing atmospheric EC measurements, we followed the energy flux paths within the water by performing extensive measurements of waves, currents and turbulence. We analyzed the partitioning of kinetic energy into different types of flows and its fate from generation to dissipation as a function of wind speed, lake size and vertical density stratification. As a result, we provide a comprehensive mechanistic understanding of energy flux paths in small lakes and regulated rivers in relation to atmospheric forcing. The project findings will improve current capabilities for modelling and predicting lake-atmosphere interactions and will contribute to a number of up-to-date research questions in biogeochemistry and freshwater ecology.
Publications
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(2022). Bulk Transfer Coefficients Estimated from Eddy-Covariance Measurements over Lakes and Reservoirs
Guseva, S., Armani, F., Desai, A.D., Dias, N., Eugster, W., Iwata, H., Friborg, T., Jansen, J., Lükő, G., Mammarella, I., Repina, I., Rutgersson, A., Sachs, T., Scholz, K., Spank, U., Stepanenko, V., Torma, P., Vesala, T., and Lorke, A.
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(2021). Energy Flux Paths in Lakes and Reservoirs. Water, 13(22), 3270
Guseva, S., Casper, P., Sachs, T., Spank, U., & Lorke, A.
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(2021). Variable Physical Drivers of Near-Surface Turbulence in a Regulated River. Water Resources Research, 57(11), e2020WR027939
Guseva, S., Aurela, M., Cortés, A., Kivi, R., Lotsari, E., MacIntyre, S., . . . Lorke, A.
