Turbulence and Gas Transfer Velocities Under Light Winds in a Small Boreal Lake and a Large Tropical Reservoir
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. The hydrodynamics of tropical water bodies have been studied more frequently, yet measurements of near-surface turbulence within them are rare. We instrumented a humic, 4.7 ha, boreal lake with 2 meteorological stations, 3 thermistor arrays, an infra-red (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 (SCAMP) used in rising mode, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). We conducted similar measurements in a large tropical reservoir. Near-surface ε varied from 10-8 m2 s-3 to 10-6 m2 s-3 for the 0 to 4 m s-1 winds over the small lake and followed predictions from Monin-Obukhov similarity theory (MOST). Near-surface ε measured with the SCAMP had similar values of ε in the tropical reservoir during light winds and cooling. Values ranged from 10-7 m2 s-3 to 10-5 m2 s-3 during diurnal heating when winds were light, near-surface eddies were less than 0.08 m in size and buoyancy frequencies ranged from 8 to 25 cycles per hour (cph). The increase in ε was only partially explained using scaling from MOST. When arithmetically averaged for measured winds less than 3 m s-1, ε was independent of wind speed. k computed from a surface renewal model using ε agreed with values from chambers and, for the small lake, increased linearly with wind speed. kcomputed from measured dissipation rates for the tropical reservoir during heating were ~10 cm hr-1 with values 40% to 100% higher than expected from MOST. Results extend scaling approaches developed in the laboratory, for large water bodies, and the atmosphere, illustrate turbulence and k are greater than expected in low wind environments, and provide new equations to quantify fluxes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMB018.0014M
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0458 Limnology;
- BIOGEOSCIENCES;
- 0495 Water/energy interactions;
- BIOGEOSCIENCES