An Intercomparison of Current Measurements Using a Vector Measuring Current Meter, an Acoustic Doppler Current Profiler, and a Recently Developed Acoustic Current Meter

Horizontal current measurements were obtained simultaneously with a vector measuring current meter (VMCM), an acoustic Doppler current profiler (ADCP), and the recently developed Falmouth Scientific, Inc., acoustic current meter (ACM) during a 110-day period from the Bermuda Testbed Mooring (BTM). The BTM site is located approximately 80 km southeast of Bermuda at 31°44′N, 64°10′W in waters of about 4550-m depth. The ACM is a relatively small, lightweight instrument, which can be deployed relatively close to the ocean surface or bottom as well as at intermediate depths. Prior to the present study, in situ data and knowledge concerning the ACM performance were quite limited. Zonal, meridional, and horizontal current speed data were obtained and time series were computed for each instrument. Several analyses were done to evaluate and intercompare instrument performance. The ACM results compared favorably with those of the VMCM and ADCP for 36-h filtered data. The ADCP typically recorded the largest horizontal current speed and the ACM typically recorded the smallest values. The strongest correlation was seen between the VMCM and ADCP data;however, the lowest correlation for all intercomparisons (by component and speed) was r = 0.95 with most values of r = 0.98 or greater. Directional error/bias is a primary factor in limiting current component measurement accuracy. In particular, the ACM direction was up to 20°–30° offset from those of the other two instruments. After rotating or “correcting” the ACM components with respect to the VMCM direction, ACM mean current component differences with respect to the other instruments’ values were less than about 2 cm s−1. Based upon the spectral analysis, both the ACM and VMCM have better performance characteristics than the ADCP in the higher-frequency portion of the spectral domain. The authors’ analysis indicates that the three different systems provide generally similar results in terms of coherence and phase with significant departures occurring only with respect to the ADCP at frequencies greater than 0.01 cph. The present study indicates that the ACM provides another viable means of obtaining accurate current measurements.