Air-water exchange and energy flux paths in small lakes

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Wind-induced water motions are an important physical characteristics of lakes and play a vital role in their ecology and biogeochemistry. 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 as well as the rate of evaporation. The processes which 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 compile recently conducted atmospheric eddy-covariance (EC) measurements of momentum, heat, water vapor and gas fluxes over 10 different small lakes. This unique data set will be used to analyze the dependence of the momentum transfer from wind to water on wind speed and fetch length and to derive mechanistic relationships between the different transfer coefficients. The energy flux paths within lakes will be studied by complementing ongoing atmospheric EC measurements with extensive measurements of waves, currents and turbulence in three lakes. We will analyze 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 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.