Echo intensity data as a directional antenna for observing processes above sloping ocean bottoms |
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Authors: | Hans van Haren |
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Institution: | (1) Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, The Netherlands |
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Abstract: | Relative ‘echo intensity’ data (dI) from a bottom-mounted 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. |
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Keywords: | ADCP echo intensity observations Near-bottom processes Continental slope Propagation and particle speeds Bay of Biscay |
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