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ADCP estimates of Reynolds and bottom stress
M.J. Howarth
Status: Accepted
Bidston Observatory
Bidston Hill
Prenton , Wirral United Kingdom
CH43 7RA
Phone: + 44 151 653 8633
Email: mjh@pol.ac.uk
Co-Authors:
Field measurements of Reynolds and bottom stress are rare and difficult
to make but are required for studies of large scale dynamical balances
and of small scale dissipation and exchange processes, including the concept
of the constant stress layer. Recently a simple to apply technique has
been reported in which Reynolds stress profiles can be calculated from
the variance of along beam data recorded by fast sample, O(1 Hz), high
frequency Acoustic Doppler Current Profilers (ADCPs).
The technique has been applied in seven experiments in the north-west
European continental shelf seas involving the deployment of 0.6 and 1.2
MHz standard broadband ADCPs mounted in sea-bed frames. The sites ranged
from high tidal energy, shallow (20 m deep) to low tidal energy, deeper
(110 m). The ADCPs recorded data with a variety of sample regimes, from
2 Hz (recording every ping) to 1 Hz (averaging 8 pings) to 0.5 Hz (averaging
over four or five pings); bin sizes ranged from 0.25 to 1 m. For most
of the experiments the ADCP estimates of shear production of turbulence
were complemented by estimates of turbulence dissipation obtained from
repeated deployments from a ship of the FLY microstructure profiler.
In two of the experiments the ADCP near bed Reynolds / bottom stress estimates
were tested against independent estimates from toroidal electro-magnetic
current meters measuring the three components of current (vertical and
both horizontal) at 8 Hz. These current meters were fitted to a frame,
at 0.3, 0.6 and 0.9 m above the bed, deployed near the ADCP. In all cases
the correlation coefficient squared between the two sets of Reynolds stress
estimates was 0.7. In addition the three components of turbulence intensity
(and hence the degree of anisotropy) were estimated from the electro-magnetic
current meter records. This cannot be estimated from the ADCP data, unless
the ADCP is fitted with a vertical beam, but is important for interpretation
of the results.
One objective of these studies is to improve representation of processes
in 2- and 3-dimensional numerical models and to test the results. At its
very simplest bottom stress can be related to the depth-averaged flow
via a quadratic drag coefficient estimates from the different sites
were all in the region of 0.001, smaller than the value used in most depth-averaged
numerical models (about 0.0025). Three avenues are being explored to understand
this result
a) corroborating evidence is being sought, mainly from bulk calculations.
b) possible errors either inherent in the technique or related to the
ADCP set up (bin size, sample interval, tilt) are being investigated and,
where possible, quantified.
c) physical reasons for the discrepancy are being considered.
Submitted on November 11, 2002
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