Preprints
https://doi.org/10.5194/os-2021-71
https://doi.org/10.5194/os-2021-71

  26 Jul 2021

26 Jul 2021

Review status: a revised version of this preprint is currently under review for the journal OS.

Impact of ADCP motion on structure function estimates of turbulent kinetic energy dissipation rate

Brian Daniel Scannell, Yueng-Djern Lenn, and Tom P. Rippeth Brian Daniel Scannell et al.
  • School of Ocean Sciences, Bangor University, Menai Bridge, Ynys Môn, LL59 5AB, United Kingdom

Abstract. Turbulent mixing is a key process in the transport of heat, salt and nutrients in the marine environment, with fluxes commonly derived directly from estimates of the turbulent kinetic energy dissipation rate, ϵ. Time series of ϵ estimates are therefore useful in helping to identify and quantify key biogeochemical processes. Estimates of ϵ are typically derived using shear microstructure profilers, which provide high resolution vertical profiles, but require a surface vessel, incurring costs and limiting the duration of observations and the conditions under which they can be made. The velocity structure function method can be used to determine time series of ϵ estimates using along-beam velocity measurements from suitably configured acoustic Doppler current profilers (ADCP). Shear in the background current can bias such estimates, therefore standard practice is to deduct the mean or linear trend from the along-beam velocity over the period of an observation burst. This procedure is effective if the orientation of the ADCP to the current remains constant over the burst period. However, if the orientation of a tethered ADCP varies, a proportion of the velocity difference between bins is retained in the structure function and the resulting ϵ estimates will be biased. Long-term observations from a mooring with three inline ADCP show the heading oscillating with an angular range that depends on the flow speed; from large, slow oscillations at low flow speeds to smaller, higher frequency oscillations at higher flow speeds. The mean tilt was also determined by the flow speed, whilst the tilt oscillation range was primarily determined by surface wave height. Synthesised along-beam velocity data for an ADCP subject to sinusoidal oscillation in a sheared flow indicates that the retained proportion of the potential bias is primarily determined by the angular range of the oscillation, with the impact varying between beams depending on the mean heading relative to the flow. Since the heading is typically unconstrained in a tethered mooring, heading oscillation is likely to be the most significant influence on the retained bias for a given level of shear. Use of an instrument housing designed to reduce oscillation would mitigate the impact, whilst if the shear is linear over the observation depth range, the bias can be corrected using a modified structure function method designed to correct for bias due to surface waves.

Brian Daniel Scannell et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on os-2021-71', Leon Boegman, 11 Aug 2021
    • AC1: 'Reply on CC1', Brian D Scannell, 12 Aug 2021
  • RC1: 'Comment on os-2021-71', Anonymous Referee #1, 02 Sep 2021
    • AC2: 'Reply on RC1', Brian D Scannell, 20 Sep 2021
  • RC2: 'Comment on os-2021-71', Anonymous Referee #2, 26 Oct 2021
    • AC3: 'Reply on RC2', Brian D Scannell, 28 Oct 2021

Brian Daniel Scannell et al.

Brian Daniel Scannell et al.

Viewed

Total article views: 547 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
435 92 20 547 4 3
  • HTML: 435
  • PDF: 92
  • XML: 20
  • Total: 547
  • BibTeX: 4
  • EndNote: 3
Views and downloads (calculated since 26 Jul 2021)
Cumulative views and downloads (calculated since 26 Jul 2021)

Viewed (geographical distribution)

Total article views: 509 (including HTML, PDF, and XML) Thereof 509 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 27 Nov 2021
Download
Short summary
Turbulent mixing is crucial in the aquatic environment; regulating the water column structure and the availability of nutrients to support ecosystems. Improving our understanding how mixing varies with changes in weather, tides and wave conditions is therefore a key challenge. This paper examines how to avoid and, where necessary, correct for bias due to instrument motion when using a relatively new technique for making long-term observations of turbulence using a moored current meter.