In-situ comparisons of moored acoustic Doppler profilers with conventional VACM and VMCM current meters

Experiments in Massachusetts Bay, the North Atlantic and the Northern California Shelf are examined where 150 and 300 kHz RD Instruments acoustic Doppler current profilers (ADCPs) were deployed near vector averaging (VACM) and vector measuring (VMCM) current meters. The ADCPs were deployed in-line on surface moorings, in both upward and downward looking configurations. Comparisons of ADCP velocities with those of VACMs and VMCMs were used to evaluate the quality of the ADCP data obtained in the surface-moored configuration, and to identify any systematic differences between the ADCP and VACM/VMCM observations. Unlike conventional current meters, the ADCPs were limited by instrumental noise at high frequencies. Regressions of ADCP versus VMCM or VACM speeds showed typical mean differences of 1 to 2 cm/sec and rms differences of about 3 cm/sec. These speed differences are similar to comparisons between conventional current meters. Comparison of ADCP and VACM/VMCM directions showed the direction difference to be a function of direction. The maximum mean difference was 14°. These directional differences (attributed to flux-gate compass enters) appear to be the limiting factor in both ADCP and conventional velocity measurements