Intercomparison tests of moored current measurements in the upper ocean

Abstract

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 20 m s -I 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 mount 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.ing buoys needs to be estimated. In this paper, current mea. surements recorded during the Mixed Layer Experiment (Mile) and the Joint Air-Sea Interaction (Jasin) project, when wind speeds and significant wave heights reached 20 m s -• and 5 m (i.e., when mooring motions were expected to be large), are used to investigate the relative performance of different kinds of current meters. VACM, acoustic current meter (ACM), and vector-measuring current meter (VMCM) (described in the instrumentation section) measurements recorded beneath surface-following buoys are compared to similar measurements recorded at approximately the same depth from spar buoys or subsurface moorings to describe the influence of mooring motion upon each type of measurement. In addition, the in situ differences between a VACM, an ACM, and a VMCM mounted on the same mooring are determined. EXPERIMENTS The 19-day Mile observations, which occurred in water about 4200 m deep at ocean weather station P (50øN, 145øW), began August 19, 1977. The average wind speed (measured from the R.V. Oceanographer) and significant wave height (measured with a waverider buoy) were 10.0 m s -• and 2.2 m, respectively (Figure 1), and at 8 m depth the vector-mean current speed was 6 cm s -•. During the initial 5-day period when all moorings remained on-station, an intense low (•992 mbar) atmospheric pressure system moved slowly over the experimental site. For about 2 days before the arrival of the storm, the average ...

Comparison of profiling and moored current measurements in the equatorial Pacific

“…The accuracy of MCM data has been fairly well established and is expected to vary little from one deployment to another. Halpern et al [1981] found that at frequencies below 0.3 cph the amplitudes of velocity fluctuations recorded by a VACM suspended beneath a surface-following buoy were 5-15% larger than corresponding measurements made beneath a spar buoy or subsurface buoy. This result, obtained in the upper 100 m at mid-latitudes, is probably an upper bound for measurements within the EUC, where mean currents are strong and mooring motion is moderate.…”

Recent long‐term moored instrument observations in the western North Pacific

“…The offdiagonal horizontal Reynolds' stresses from two numerical experiments previously discussed by Schmitz and Holland [1982] are compared with observation in section 5, followed by summary and conclusions (section 6). Halpern et al [1981]. A general review of mooring design and performance may be found in Berteaux [1976].…”

Moored observations of current and temperature on the shelf and upper slope near ring 82B

“…Twelve vector-averaging current meters (VACMs) were deployed in the array; seven VACMs were at a common depth of 10 m on surface moorings, and five VACMs were at 40 m and deeper levels on subsurface moorings. Current meter intercomparisons such as CMICE [Beardsley et al, 1981] and MILE [Halpern et al, 1981] indicate that the contamination of the 10-m current measurements by the wave-induced motion of the surface flotation is minimized by the slack surface mooring configuration and by the internal high-frequency vector averaging in the VACM: the expected experimental error in the 10-m current observations is less than 10%, especially during summer when the surface wave energy is at a minimum. A cross-sectional view of the array is shown in Figure 2 along with the numerical code used to refer to the instruments in the text and figures.…”