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 () 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 profiles from ϳ5 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, and P are expected to covary with mean values that are equal. The results show a close tracking of 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 /P Ӎ 0.63 Ϯ 0.17 was obtained. Although this is a significant deviation from unity, it is within the range of uncertainty previously reported for the measurements. A marked phase lag of between 5 and 20 min between the maximum P and the maximum 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 P min ϳ 7 ϫ 10 Ϫ5 W m Ϫ3 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 (1Њ-3.5Њ) 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.

Effect of variable winds on current structure and Reynolds stresses in a tidal flow: analysis of experimental data in the eastern English Channel

“…As the consequence, during storm periods, particularly when the wind blows against the ebbing tide, we observed a significant enhancement of turbulence while, during the flood flow, turbulence generation near the sea surface was moderate. Earlier, a similar effect was reported by Seim (2002) and Rippeth et al (2003).…”

“…Since applying oceanographic ADCP (an Acoustic Doppler Current Profiler) measurements by Lohrmann et al (1990), the Variance Fit method (VM) has been used successfully in a large number of studies of energetic tidal systems (Lu and Lueck, 1999a, b;Stacey et al, 1999;Rippeth et al, 2002Rippeth et al, , 2003Fugate and Chant, 2005;Souza and Howarth, 2005;Nidzieko et al, 2006;Peters and Johns, 2006;Korotenko and Sentchev, 2011). However, in the presence of energetic surface gravity waves, the prediction of turbulent quantities with VM presents certain difficulties.…”

“…Since surface waves often occupy the same frequency range as marine turbulence, it is difficult to separate the latter from waveinduced velocity fluctuations using simple filtration. Therefore, development of various techniques and methods capable to remove the bias produced by surface waves from ADCP measurements of turbulent shear stress was an important issue over the past decade (Shaw and Trowbridge, 2001;Trowbridge and Elgar, 2003;Whipple et al, 2006;Feddersen and Williams, 2007;Rosman et al, 2008;Schmitt et al, 2009;Huang et al, 2010;Kirincich et al, 2010). This paper addresses two challenges.…”

Reynolds stress observations in continental shelf seas

“…The water depths, tidal current speeds and record durations are listed in Table 2. Site 7 was in the Gulf of California (Souza et al, 2004), site 2 in the northern North Sea, site 3 in the southern North Sea, and the remainder in the Irish Sea-site 1 in Red Wharf Bay, off the east coast of Anglesey (Rippeth et al, 2003), sites 4 and 5 in Liverpool Bay (Souza and Howarth, 2005), site 6 off Holyhead, west of Anglesey (see Fig. 1 for a map of the sites around the UK).…”

An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes

“…These techniques allow for the direct measurements of turbulent properties, such as kinetic energy and Reynolds (shear) stresses at a specific point in the water column (e.g., van der Ham 1999). More recently, studies by Stacey et al (1999) and Rippeth et al (2003) have employed the variance method (Tropea 1983;Lohrmann et al 1990) to determine Reynolds shear stresses with acoustic Doppler current profilers (ADCPs) over the whole water column. However, this measurement technique does not allow for direct measurements of the turbulent buoyancy flux.…”