Turbulence in a small arctic pond

Abstract

Small ponds, numerous throughout the Arctic, are often supersaturated with climate-forcing trace gases. Improving estimates of emissions requires quantifying (1) their mixing dynamics and (2) near-surface turbulence which would enable emissions. To this end, we instrumented an arctic pond (510 m2, 1 m deep) with a meteorological station, a thermistor array, and a vertically oriented acoustic Doppler velocimeter. We contrasted measured turbulence, as the rate of dissipation of turbulent kinetic energy, ε, with values predicted from Monin–Obukhov similarity theory (MOST) based on wind shear as u*w, the water friction velocity, and buoyancy flux, β, under cooling. Stratification varied over diel cycles; the thermocline upwelled as winds changed allowing ventilation of near-bottom water. Near-surface temperature stratification was up to 7°C per meter. With respect to predictions from MOST: (1) With positive β under heating and strong near-surface stratification, turbulence was suppressed; (2) under heating with moderate stratification and under cooling with light to moderate winds, measured ε was in agreement with MOST; (3) under cooling with no wind and when surface currents had ceased, as occurred 20% of the time, turbulence was measurable and predicted from β. Near-surface turbulence was enhanced under cooling and light winds relative to that under a neutral atmosphere due to higher values of drag coefficients under unstable atmospheres. Small ponds are dynamic systems with wind-induced thermocline tilting enabling vertical exchanges. Near-surface turbulence, similar to that in larger systems, can be computed from surface meteorology enabling accurate estimates of gas transfer coefficients and emissions.