LAGRANGIAN MEASUREMENTS OF TURBULENT DISSIPATION OVER A SHALLOW TIDAL FLAT FROM PULSE COHERENT ACOUSTIC DOPPLER PROFILERS

[摘要] We present high resolution (25 mm spatial, 8 Hz temporal) profiles of velocity measured over a shallow tidal flat using pulse-coherent Acoustic Doppler Profilers mounted on surface drifters.The use of Lagrangian measurements mitigated the problem of resolving velocity ambiguities, a problem which often limits the application of high-resolution pulse-coherent profilers.Turbulent dissipation rates were estimated from second-order structure functions of measured velocity.Drifters were advected towards, and subsequently trapped on, a convergent surface front which marked the edge of a freshwater plume.Measured dissipation rates increased as a drifter deployed within the plume approached thefront.A drifter then propagated with and along the front as the fresh plume spread across the tidal flats.Near-surface turbulent dissipation measured at the front roughly matched a theoretical mean-shear-cubed relationship, whereas dissipation measured in the stratified plume behind the front was suppressed.After removal of estimates affected by surface waves, near-bed dissipationmatched thevelocity cubed relationship, although scatter was substantial.Dissipation rates appeared to be enhanced when the drifter propagated across small subtidal channels.

[学科分类] 建筑学

[关键词] drifters;turbulent dissipation;Lagrangian measurements;shallow flows;structure functions;pulse coherent profilers;tidal flats