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

Abstract

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.