Abstract
The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air–water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate‐forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (ε) using an acoustic Doppler velocimeter and a temperature‐gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near‐surface ε varied from 10−8 to 10−6 m2 s−3 for the 0–4 m s−1 winds and followed predictions from Monin–Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10−3 m2 s−1 on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L N) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using ε agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s−1, a condition that can lead to elevated near‐surface ε and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind‐sheltered lakes, and provide new equations to quantify fluxes.
Highlights
Turbulent processes in the water column are quantified with the coefficient of eddy diffusivity (Kz), those at the air–water interface by gas transfer velocities (k), and horizontal spreading by a dispersion coefficient (KH)
Our goal is to describe the hydrodynamics of the small lake to inform studies of lake metabolism and greenhouse gas evasion
We quantify near-surface turbulence using temperature-gradient microstructure data and data from an acoustic Doppler velocimeter, and we evaluate how well a recently derived similarity scaling for lakes predicts near-surface turbulence
Summary
MOST has been found to apply within water bodies over a range of sizes (Lombardo and Gregg 1989; Tedford et al 2014; MacIntyre et al 2018) but has not been tested in boreal lakes These equations allow turbulence, as the rate of dissipation of turbulent kinetic energy, ε, to be estimated at the air–water interface, as needed for gas transfer velocities, and throughout the upper mixing layer to calculate the coefficient of eddy diffusivity (MacIntyre et al 2018). At a given depth z, for instance near the surface where gas exchange occurs, a time series of z/LMO illustrates when shear is the driver, and law of the wall scaling applies, and when buoyancy flux augments turbulence production Testing of these equations, which are based on readily measured variables such as wind speed, relative humidity, and air and surface water temperature, can be done using instrumentation that directly measures turbulence, such as acoustic Doppler velocimeters and microstructure profilers. F is the product of the gas transfer velocity (k) and the concentration gradient across the thin layer on the water side of the air–water interface: À
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