Echo intensity data as a directional antenna for observing processes above sloping ocean bottoms

Abstract

Relative 'echo intensity' data (dI) from a bottommounted four-beam 300 kHz acoustic Doppler current profiler (ADCP) are used to infer propagation of vigorous processes above a continental slope. The 3-to 60-m horizontal beam spread and the 2-Hz sampling allow the distinction of different arrival times t i , i=1,..., 4, at different distances in the acoustic beams from sharp changes in dI-content associated with frontal non-linear and turbulent bores or 'waves'. The changes in dI are partially due to variations in amounts of resuspended material carried by the near-bottom turbulence and partially due to the fast variations in density stratification ('stratified turbulence'), as inferred from 1-Hz sampled thermistor string data above the ADCP. Such bores are observed to pass the mooring up to 80 m above the bottom, having typical propagation speeds c=0.15-0.5 m s −1 , as determined from dI(t i ). Particle speeds in the immediate environment of a bore amount to |u| env =c±0.05 m s −1 , the equality being a necessary condition for kinematic instability, whilst the maximum particle speeds amount |u| max =1.2-2c. The dI-determined directions of up-, down-and alongslope processes are all to within ±10°of the ADCP's beam-spread averaged current (particle velocity) data.

A downslope propagating thermal front over the continental slope

“…It has not been calibrated and cannot provide a quantitative measure for (fine) sediment resuspension. The ADCP measured at a rate once per 15 s, which is here sufficient to obtain detailed information on phase propagation of fronts from the different arrival times in echo amplitudes of the beams [ van Haren , ]. From that previous experiment by van Haren [], in which frontal phase speed c and direction estimates from echo amplitudes are compared with particle velocities u p , we know that they are highly aligned in direction within a range ±10°, while | u p | max = 1.6 ± 0.4 c and the frontal bores' particle speed | u p | enc ≈ c .…”

“…The ADCP measured at a rate once per 15 s, which is here sufficient to obtain detailed information on phase propagation of fronts from the different arrival times in echo amplitudes of the beams [ van Haren , ]. From that previous experiment by van Haren [], in which frontal phase speed c and direction estimates from echo amplitudes are compared with particle velocities u p , we know that they are highly aligned in direction within a range ±10°, while | u p | max = 1.6 ± 0.4 c and the frontal bores' particle speed | u p | enc ≈ c . We can therefore use the information on the directly measured particle velocities from ADCP to establish speed and direction of frontal propagation and compare with the inferred phase speed and direction of the front from the different times of elevated acoustic backscatter in opposing beams.…”

“…The relative acoustic echo amplitude “dI” is computed with respect to its time mean for every 2 m depth level (vertical bin) of the ADCP. The 300 kHz dI gives some qualitative indication of resuspended sediment near sloping topography [ Hosegood et al ., ], but we note that this may be partially mixed with stratified turbulence there [ van Haren , ] while being most sensitive to “particles” of sizes of a mm and larger [ RDI , ]. It has not been calibrated and cannot provide a quantitative measure for (fine) sediment resuspension.…”

Analysis of long time series of ADCP backscatter data in the Ligurian Sea to investigate the zooplankton variability

“…Improve the knowledge on DVM is important not only for the marine biologists but also for the physical oceanographers, given that this signal may produces a noise on the vertical current measurements [1] [2][3] [4][5] [6]. A great contribution to the investigations on this topic, may be given by the Acoustic Doppler Current Profiler (ADCP).…”

Insights from a 3‐D temperature sensors mooring on stratified ocean turbulence

“…This drop was not attributed to instrumental deficiency but to nonlinear wave effects [ Briscoe , ]. A set of 2 Hz sampled acoustic Doppler current profiler (ADCP) observations from halfway up the continental slope in the Bay of Biscay confirmed these decorrelation scales for highly nonlinear turbulent bores that dominate internal wave breaking above sloping topography [ van Haren , ].…”