Turbulence at the Air-Water Interface in Lakes of Different Sizes: Consequences for Gas Transfer Coefficients

Monday, 15 December 2014
Sally MacIntyre1, Adam Timothy Crowe1, Joao H. Amaral2, Lars Arneborg3, David Bastviken4, Bruce R. Forsberg2, John M. Melack1, Julio Tota5, Edmund W Tedford1, Jan Karlsson6, Eva Podgrajsek7, Andreas Andersson7 and Anna Rutgersson7, (1)University of California Santa Barbara, Santa Barbara, CA, United States, (2)Instituto Nacional de Pesquisas da Amazônia (INPA), Manaus, Brazil, (3)University of Gothenburg, Dept. of Earth Sciences and Oceanography, Gothenburg, Sweden, (4)Linköping University, Linköping, Sweden, (5)UEA / INPA / SUNY, Manaus, Brazil, (6)Umeå University, Umeå, Sweden, (7)Uppsala University, Uppsala, Sweden
Similarity scaling predicts that wind induced shear will be the dominant source of turbulence near the air-water interface in lakes with low to moderate wind forcing. Turbulence is expected to be enhanced with wave activity; results are conflicting on the effects of heating and cooling. We measured turbulence with an acoustic Doppler velocimeter (ADV) and / or a temperature-gradient microstructure profiler and obtained correlative time series measurements of meteorology and water column temperature in a 800 m2 arctic pond, a 1 ha boreal lake, and a large tropical reservoir. Turbulence measurements with both instruments corroborated those calculated from similarity scaling in the boreal lake. Within the arctic pond, dissipation rates obtained with the ADV were in agreement with those from similarity scaling when winds exceeded ~1.5 m/s with a greater frequency of measurable dissipation rates when surface waves were present. Dissipation rates in the tropical reservoir reached and often exceeded 10‑6 m2 s-3 in the upper meter under light winds and decreased by an order of magnitude with cooling or rainfall. Under cooling, dissipation rates were at least an order of magnitude higher in the uppermost 25 cm bin than in the water column below. Gas transfer coefficients calculated from concurrent measurements of greenhouse gas fluxes with floating chambers and the surface renewal model using the estimates of turbulence were in agreement. These results support the predictions of Monin-Obuhov similarity scaling in that shear dominates turbulence production near the air-water interface under heating and cooling, illustrate spatial variability in turbulence production in small water bodies due to the intermittency of wind interacting with the water’s surface, are in agreement with prior oceanic observations that shear and associated turbulence can be intensified in shallow mixing layers under heating with light winds, and illustrate the utility of similarity scaling for predicting gas transfer coefficients.