Estimates of Reynolds and bottom stress from fast sample ADCPs deployed in continental shelf seas

Reynolds stress profiles, and hence estimates of bottom stress and of profiles of the shear production of turbulence, 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 six experiments in the northwest 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 0.5 Hz (averaging over four or five pings); bin sizes ranged from 0.25 to 1 m. The results from two of the experiments are presented, in which a bottom frame fitted with electro-magnetic current meters measuring the three components of current (vertical and both horizontal) at 8 Hz at three levels (0.3, 0.6 and 0.9 m above the bed) was also deployed near to the ADCPs. The correlation coefficient between estimates of Reynolds stress from the ADCP data and from the electro-magnetic current meter records was 0.8 in all cases. 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 ADCP data, unless the instrument is fitted with a vertical beam, but is valuable for interpretation of the results.