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

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. 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.

Anthropogenic Mixing in Seasonally Stratified Shelf Seas by Offshore Wind Farm Infrastructure

“…The potential power added to the thermocline, P D , varies between approximately 2 × 10 −6 W m −3 to 4 × 10 −4 W m −3 with an average value of 5.9 × 10 −5 W m −3 . Over the same period, at the same site, the natural background dissipation rate (ε B ) at the thermocline is 3.5 × 10 −5 W m −3 (Scannell et al, 2021). In this approximate calculation, the additional (average) turbulence production is therefore 69% greater than background dissipation without monopiles present:…”

Anthropogenic Mixing of Seasonally Stratified Shelf Seas by Offshore Wind Farm Infrastructure

“…Simpson et al, 1982). Shallow water model studies of the effects of offshore wind farms on local oceanic circulation patterns have shown arrays of infrastructure induce strong horizontal shear in (Scannell et al, 2021). Turbulence production Eq.…”

“…Mean TKE production by a d = 10 m offshore monopile. TKE production is based on direct velocity measurements averaged over the thermocline, 20 m to 50 m in the Celtic Sea between 22/06/2014 and 20/08/2014, Figure 3(Scannell et al, 2021). Turbulence production Eq.…”