Vector Measuring Current Meter Research Articles

Four Current Meter Models Compared in Strong Currents in Drake Passage

Abstract Seven current meters representing four models on a stiffly buoyed mooring were placed for an 11-month deployment to intercompare their velocity measurements: two vector-measuring current meters (VMCMs), two Aanderaa recording current meter (RCM) 11s, two Aanderaa SEAGUARDs, and a Nortek Aquadopp. The current meters were placed 6-m apart from each other at about 4000-m depth in an area of Drake Passage expected to have strong currents, nearly independent of depth near the bottom. Two high-current events occurred in bursts of semidiurnal pulses lasting several days, one with peak speeds up to 67 cm s−1 and the other above 35 cm s−1. The current-speed measurements all agreed within 7% of the median value when vector averaged over simultaneous time intervals. The VMCMs, chosen as the reference measurements, were found to measure the median of the mean-current magnitudes. The RCM11 and SEAGUARD current speeds agreed within 2% of the median at higher speeds (35–67 cm s−1), whereas in lower speed ranges (0–35 cm s−1) the vector-averaged speeds for the RCM11 and SEAGUARD were 4%–5% lower and 3%–5% higher than the median, respectively. The shorter-record Aquadopp current speeds were about 6% higher than the VMCMs over the range (0–40 cm s−1) encountered.

Evaluating Radial Current Measurements from CODAR High-Frequency Radars with Moored Current Meters*

The performance of a network of five CODAR (Coastal Ocean Dynamics Application Radar) SeaSonde high-frequency (HF) radars, broadcasting near 13 MHz and using the Multiple Signal Classification (MUSIC) algorithm for direction finding, is described based on comparisons with an array of nine moorings in the Santa Barbara Channel and Santa Maria basin deployed between June 1997 and November 1999. Eight of the moorings carried vector-measuring current meters (VMCMs), the ninth had an upward-looking ADCP. Coverage areas of the HF radars and moorings included diverse flow and sea-state regimes. Measurement depths were ∼1 m for the HF radars, 5 m for the VMCMs, and 3.2 m for the ADCP bin nearest to the surface. Comparison of radial current components from 18 HF radar–mooring pairs yielded rms speed differences of 7–19 cm s−1 and correlation coefficients squared (r2) in the range of 0.39–0.77. Spectral analysis showed significant coherence for frequencies below 0.1 cph (periods longer than 10 h). At higher frequencies no significant coherence was found, and noise levels corresponding to 6 cm s−1 rms were evident in the radar data. Errors in the radar bearing determination were found in 10 out of 18 comparisons, with a typical magnitude of 5°–10°, and a maximum of 19°. The effects of bearing errors on total vector currents were evaluated using a simple flow field and measured bearing errors, showing up to 15% errors in computed flow speeds, and up to ∼9° errors in flow directions.

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 318449N, 648109W 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 5 0.95 with most values of r 5 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 208‐308 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 21. 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.

Current Structure Variations Detected by High-Frequency Radar and Vector-Measuring Current Meters

Abstract Ocean surface current measurements from high-frequency (HF) radar are assessed by comparing these data to near-surface current observations from 1 to 30 October 1994 at two moored subsurface current meter arrays (20 and 25 m) instrumented with vector-measuring current meters (VMCMs) and Seacat sensors during the Duck94 experiment. A dual-station ocean surface current radar (OSCR) mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 25 km × 44 km domain using the HF (25.4 MHz) mode and directly overlooked these moorings. In response to wind, tidal, and buoyancy forcing over 29 days, surface current observations were acquired 95% of the time in the core of the OSCR domain, decreasing to levels of about 50% in the offshore direction. Regression analyses between surface and subsurface measurements at 4 and 6 m indicated biases of 2–6 cm s−1, slopes of O(1), and rms differences of 7–9 cm s−1. Episodic freshwater intrusions of about 30 practical salinity units (psu)...

Measurements of the water-following capability of holey-sock and TRISTAR drifters

Since 1985, a number of measurements have been made in deep water to determine the water-following characteristics of mixed layer drifters with both holey-sock and TRISTAR drogues at 15 m depth. The measurements were done by attaching two neutrally buoyant vector measuring current meters (VMCMs) to the top and the bottom of the drogues and deploying the drifters in different wind and upper ocean shear conditions for periods of 2–4 h. The average velocity of the VMCM records was taken to be a quantitative measure of the slip of the drogue through the water, observed to be 0.5-3.5 cm s −1. The most important hydrodynamic design parameter which influenced the slip of the drogue was the ratio of the drag area of the drogue to the sum of the drag areas of the tether and surface floats: the drag area ratio R. The most important environmental parameters which affected the slip were the wind and the measured velocity difference across the vertical extent of the drogue. A model of the vector slip as a function of R, vector wind and velocity difference across the drogue was developed and a least squares fit accounts for 85% of the variance of the slip measurements. These measurements indicated that to reduce the wind produced slip below 1 cm s −1 in 10 m s −1 wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s −1 accuracy, the displacement of the R = 40 drifter can be corrected to an accuracy of 0.5 km day −1.

Comparison of Buoy-Mounted 75-kHz Acoustic Doppler Current Profilers with Vector-Measuring Current Meters

Abstract Long time series of atmospheric parameters and limited oceanographic parameters such as near-surface temperature and wave statistics have been available for some time. There is, however, a need for similar observations of currents in the coastal ocean. In December 1991, the National Data Buoy Center (NDBC) deployed two meteorological buoys in the Southern California Bight on a transect between San Diego and San Clemente Island. Each buoy consisted of a 10-m discus hull instrumented to measure a suite of meteorological parameters, and, for the first time in the NDBC buoy program, acoustic Doppler current profilers (ADCPs) were included to gather hourly current profiles beneath the two buoys. Moorings instrumented with seven vector-measuring current meters (VMCMs) were deployed adjacent to the NDBC buoys for several months and provided current observations for comparison with the ADCP measurements. When the situation is such that the wave-induced buoy motion is not overly large, the observations of...

High-speed real-time data acquisition for vector measuring current meters

The authors describe an integrated hardware and software approach used to provide real-time acquisition and display of vector measuring current meter (VMCM) data at high sample rates (0.5 Hz) with supporting data from other instruments of somewhat slower rates. The hardware component consisted of a new circuit board for the VMCM that captured the VMCM data output, stored it, and transmitted it at high speed in response to a request from the data acquisition computer. The data acquisition software ran on a Masscomp 5600 computer under the UNIX operating system. A central control program provided timing information to several software processes that handled serial data acquisition, display, and storage for one or more instruments. Deployed as a part of the Surface Waves Processes Program (SWAPP) experiment, the integrated data acquisition system successfully collected, processed, stored, and displayed data from 19 VMCMs and four other instruments over a 22-day period in February and March of 1990. >

Measuring Upper Ocean Variability from an Array of Surface Moorings in the Subtropical Convergence Zone

Abstract Measurements of upper ocean variability were made in the subtropical convergence zone southwest of Bermuda from an array of five surface moorings set with spacings of 16 to 53 km. The intent was to observe oceanic fronts and to quantify the spatial gradients associated with them. Vector Measuring Current Meters (VMCMS) and Vector Averaging Current Meters (VACMS) were attached to the mooring lines beneath the surface buoys to measure velocities and temperatures. Modifications were made to the VMCMs in an attempt to improve data return. The performance and accuracy of these moored instruments are examined. Predeployment and postdeployment calibrations were carried out; and other sources of error, such as mooring motion, are considered. A number of oceanic fronts passed through the moored array during the experiment, and the horizontal gradients observed in the velocity and temperature fields were significantly larger than the uncertainties in measuring those gradients.

Water-following characteristics of a mixed layer drifter

Accurate measurements of mean Lagrangian displacements of water parcels in the mixed layer are difficult to make because wave forces on objects that float on or near the ocean surface can be easily rectified to produce a unidirectional motion. Here we discuss a design for an ARGOS-tracked drifter that is configured so as to minimize surface wave effects, and present measurements of its water-following characteristics in the California Current system in July 1985 at 15 m nominal depth in a small range of wind and wave conditions. The unique features of the drifter are small, spherically symmetric, surface and subsurface floats (to reduce directional wave forces) and a large, semi-rigid, three-axis symmetric drogue in the shape of a corner-radar reflector (to reduce kiting in shear) that self-deploys from a folded configuration. Relative 1–2 h mean flows of 0.6–2.9 cm s −1 past the drogue were measured with two vector-measuring current meters (VMCM's) attached to the top and bottom of the drogue in the presence of significant wave heights of 0.3–2.2 m, wind speeds of 2–10 m s −1, and mean vertical shears of 0.34–3.12 × 10 −2s −1. A drag force model shows that the most significant forces in the least square sense are due to winds or surface gravity waves, jointly accounting for 74% of the variance. In July 1985, 20 drogues were released in the vicinity of 31°N, 120°W. Although the encapsuled ARGOS transmitter in the small surface sphere submerged a significant portion of time, an average of 5 out 6 possible ARGOS positions were obtained daily. Recovery and inspection of one drifter both 4 and 8 months after deployment in the California Current revealed the conditions of the mechanical components to be excellent.

Comparison of Upper Ocean VACM and VMCM Observations in the Equatorial Pacific

Abstract Measurements from a Savonius rotor and vane vector-averaging current meter (VACM) and dual orthogonal propeller vector-measuring current meter (VMCM) placed 1 m apart beneath a surface-following buoy moored on the Pacific equator for 6-month intervals are described. Experiments at 13, 98, 120 and 160 m depths were completed in light winds (≈5 m s−1), small surface wave heights (1–2 m), strong current speeds (monthly averaged speeds ≈65 cm s−1) and large current shears (0.0 1 s−1). Excellent coherence was found between each VACM-VMCM doublet's data. As expected, the near-surface VACM recorded slightly larger speeds (≈10% for monthly mean 15-min vector-averaged values) than the VMCM. The only kinetic energy density mismatch at the 95% confidence level was between the 13-14 m dyad and occurred above 1.3 cph. The most striking feature of them intercomparison tests was that the VACM and VMCM observations were virtually identical below the new-surface layer. Below 98 m the average rms amplitude differe...

A comparison of the vector‐averaging current meter and new Edgerton, Germeshausen, and Grier, Inc., vector‐measuring current meter on a surface mooring in Coastal Ocean Dynamics Experiment 1

An intercomparison experiment was conducted between a near‐surface vector‐averaging current meter (VACM) and an Edgerton, Germeshausen, and Grier, Inc., vector‐measuring current meter (VMCM) at a midshelf mooring site (38°32.8′N, 123°22.6′W) located in 90 m of water off northern California between April and July 1981. The VACM and VMCM were deployed at 9 and 11 m, respectively, beneath a 1.5‐m spherical surface buoy on a slack mooring. The VACM and VMCM 1‐hr vector‐averaged velocities were highly correlated with a complex correlation coefficient amplitude of 0.991, indicating the relative effectiveness of the internal vector‐averaging sampling scheme of the VACM and the flow sensors of the VMCM to filter much of the wave‐induced high‐frequency currents and mooring motion. The vector‐averaged VACM speeds Sa generally exceeded the VMCM speeds Sm, with a mean and maximum speed difference of 4 and 14 cm/s, respectively. Since the observed difference in vector‐averaged speeds, ΔS = Sa − Sm, was larger on average than an independent estimate of the true shear by a factor of 4 to 5, most of the difference in the observed VACM and VMCM vector‐averaged speeds has been attributed to instrumental error. The surface wave climate was measured nearby over the shelf during part of the experiment, and a significant correlation was found between estimates of the rms horizontal velocity and rms difference vertical velocity on the basis of the observed wave data and a VACM‐derived rms wave speed W, defined as (R2 − Sa2)½, where R is the hourly averaged VACM rotor speed. This result allows the fractional vector‐averaged speed difference ΔS/Sm to be determined experimentally from the field data as a function of Sm/W, a signal‐to‐noise parameter expressing the ratio of the observed VMCM vector‐averaged speed to the VACM‐derived rms wave speed W. Although significant scatter occurs between simultaneous 1‐hr vector‐averaged VACM and VMCM speeds, the average fractional speed difference ΔS/ Sm decreases from about 0.57 ± 0.10 (at 95% confidence) at Sm/W = 0.15 to about 0.20 at Sm/W = 1 and remains approximately constant at 0.16 ± 0.02 for Sm/W between about 1.5 and 2.0. Thus the fractional speed difference observed in the field is a relatively well‐defined function of the relative magnitudes of the mean horizontal and wave‐associated flow components. Comparison of these field measurements with published laboratory data on the performance of the VACM and VMCM in unsteady flow suggests that the field and laboratory measurements are consistent and can be used to estimate the absolute fractional speed error of each current meter as a function of u/w, where u is the true mean horizontal vector speed and w the amplitude of the rms wave‐induced oscillatory velocity. The combined laboratory and Coastal Ocean Dynamics Experiment 1 field data thus indicate that in an absolute sense the VACM overresponds by about 18–20% at u/w = 0.5 to about 10–11% at u/w = 2.0, while the VMCM underresponds by about −5 to −6% for u/w between 0.5 to 2.0. These results should be independent of mooring type and thus allow estimation of absolute error in velocity measurements made with VACMs and VMCMs in the range of u/w between 0.5 and 2.0.

A comparison of two current meters on a surface mooring

An EG&G Vector Measuring Current Meter (VMCM) and an Aanderaa Recording Current Meter (RCM4) were moored below a surface-following buoy. The RCM4 instrument at 4 m depth recorded currents similar to those measured by the VMCM at 2 m, in spite of rotor pumping and direction aliasing. The speeds measured by the RCM4 did show the effect of pumping by an absence of low speeds (<25 cm s −1) during most of the time series. However, the mean velocities and standard deviations of the flow field for the two instruments were similar. The major differences were for current fluctuations of frequency >0.125 cph (0.00208 Hz) where the RCM4 overestimated the energy by up to an order of magnitude. A mooring design using a surface follower buoy in strong currents (50 cm s −1) can be used with RCM4 instruments to measure accurately low frequency current fluctuations.

Intercomparison tests of moored current measurements in the upper ocean

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.

A vector measuring current meter

A vector measuring current meter (VMCM) is described. It has two orthogonal cosine response propeller sensors that directly measure components of horizontal velocity. Using a heading from a flux-gate compass the measured components are rotated into the east-west and north-south velocity components, averaged, and written to cassette tape. The instrument, the propeller sensors, and the results of extensive laboratory tests of the instrument in combinations of steady and unsteady flows are described. Comparison of laboratory tests of the VMCM with similar tests of other current meters show the VMCM to be capable of measuring small mean currents in the presence of high frequency oscillatory flows with less error than other current meters.