Use of ADCP and S4 current meters at high sampling rates in stratified Scottish coastal waters

Sea-water in the Sound of Sleat is stratified in summer. The authors measured currents in the 20 metres of the water closest to the surface with an RDI acoustic Doppler current profiler (ADCP: 1200 kHz) and a vertical moored string of InterOcean electromagnetic S4 current meters in light winds for about 100 hours in summer 1992. The frequency of averaged measurement was every minute. Comparison of the records reveals a good correlation between the types of instrument that improves as averages are taken over longer periods of a few minutes. At higher wind speed the correlation deteriorates. Spectral speed analysis shows the noisier nature of ADCP records and the greater discrimination of the S4 in detecting spectral peaks. Differences may be related to difficulties of determining a zero calibration for the S4s and to lesser sensitivity of the ADCP to the effects of small scale turbulence in the natural flows. The use of S4s to discriminate on small scales is illustrated by reference to a horizontal shear measurement with the rigid floating DML surface spar.