Abstract
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. 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 (ADCPs). 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 the 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 ADCPs 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 indicate 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.
Highlights
The most well-established technique for making observations of the turbulent kinetic energy (TKE) dissipation rate, ε, uses shear microstructure profilers (e.g. Dewey et al, 1987; Lueck et al, 2002)
An alternative approach using acoustic Doppler velocimeters (ADVs) to make point observations of the velocity spectrum has been used from a mooring, but ε estimates are subject to potentially high levels of motioninduced contamination (Bluteau et al, 2016)
For an upward- or downward-looking acoustic Doppler current profilers (ADCPs) with constant heading such that the horizontal projection of beam i is oriented into a steady, non-turbulent, vertically sheared horizontal flow with current speed U (z), the difference in the alongbeam velocity b observed between bin number j and j + n will be b(i, j ) − b(i, j + n) = sin θ n δz, (7)
Summary
The most well-established technique for making observations of the turbulent kinetic energy (TKE) dissipation rate, ε, uses shear microstructure profilers (e.g. Dewey et al, 1987; Lueck et al, 2002). The development of new ADCP operating modes such as pulse–pulse coherent and high ping rates has allowed high-spatial-resolution low-variance velocity measurements to be made without the need for extensive time averaging, but with limited beam range This has encouraged innovations such as deployments on tethered moorings to acquire turbulence measurements in sections of the water column important for mixing The aims of this paper are to demonstrate that ε estimates derived from velocity observations from the angled beams of a tethered ADCP in a sheared flow using the standard structure function method are inherently susceptible to bias if the instrument orientation to the flow varies, to highlight the key factors determining the level of such bias, and to outline possible means of mitigating or correcting for the effect.
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