Intercomparison tests of moored current measurements in the upper ocean

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Abstract
During the August‐September 1977 Mixed Layer Experiment (Mile) and the July‐September 1978 Joint Air‐Sea Interaction (Jasin) project, moored current measurements were made in the upper ocean with Savonius rotor and vane vector‐averaging current meters (VACM), dual orthogonal propeller vector‐measuring current meters (VMCM), and dual orthogonal acoustic travel‐time vector‐averaging current meters (ACM). Wind speeds and significant wave heights reached 20ms−1 and 5 m. The influence of mooring motion upon ACM, VACM, and VMCM measurements are described. In the mixed layer above about 30 m depth where mean currents are relatively large, the effect of a surface‐following buoy upon ACM, VACM, and VMCM velocity fluctuations at frequencies less than 0.3 cph was negligible; at frequencies above 4 cph, the VACM data contained the largest amount of mooring induced contamination. Below the mixed layer at depths greater than about 75 m, a subsurface mooring should be used; however, when a surface‐following buoy was used, then VMCM data better approximated the spectrum of the fluctuations than VACM data. A spar‐buoy should not be used to measure currents at depths as deep as 80 m. The frequency‐dependent differences between VACM and VMCM and between VACM and ACM measurements are described. At frequencies less than 0.3 cph, the differences between the VACM and ACM or the VMCM records were not significant with 95% confidence limits, were always positive, and above 80 m depth were less than 20%. At frequencies above 4 cph, the VACM‐VMCM differences were about 5 times larger than the VACM‐ACM differences.

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