Measurement of the rates of production and dissipation of turbulent kinetic energy in an energetic tidal flow: Red wharf bay revisited

Abstract

Simultaneous measurements of the rates of turbulent kinetic energy (TKE) dissipation (ε) and production (P) have been made over a period of 24 h at a tidally energetic site in the northern Irish Sea in water of 25-m depth. Some ε profiles from ∼5 m below the surface to 15 cm above the seabed were obtained using a fast light yoyo (FLY) microstructure profiler, while P profiles were determined from a bottom-mounted high-frequency acoustic Doppler current profiler (ADCP) using the variance method. In homogeneous flow of the kind observed, the turbulence regime should approximate to local equilibrium so that, with no buoyancy forces involved, ε and P are expected to covary with mean values that are equal. The results show a close tracking of ε and P for most of the observational period. For the second tidal cycle, when there was no significant surface wave activity, a mean ratio of ε/P ≃ 0.63 ± 0.17 was obtained. Although this is a significant deviation from unity, it is within the range of uncertainty previously reported for the ε measurements. A marked phase lag of between 5 and 20 min between the maximum P and the maximum ε is interpreted using a simple model in terms of the decay rate of TKE. Consideration of inherent instrument noise has enabled an estimate of the lowest P threshold measurable using the variance technique. For the chosen averaging parameters a value of Pmin ∼ 7 × 10-5 W m-3 is estimated. Two other significant differences between the two sets of measurements are attributed to errors in the stress estimate. The first is a bias in the estimate of stress resulting from a combination of instrument tilt (1°-3.5°) and surface wave activity. The second are anomalously high stress estimates, covering nearly one-half of the water column at times, which are thought to be due to instrument noise associated with the large wave orbital velocities.