An inter-comparison between the VACM and new EG&G VMCM on a surface mooring

An intercomparison experiment was conducted between a near-surface VACM and an EG&G VMCM at a mid-shelf mooring site 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 one-hour vector-averaged velocities were highly correlated with a complex correlation coefficient amplitude of .991, with VACM speeds generally exceeding VMCM speeds by up to 10 cm/sec. Since the difference was larger on average than an independent estimate of true shear by a factor of 4 to 5, most of the difference in observed VACM and VMCM speeds is attributed to instrumental error. Independent measurement of the surface wave climate allowed experimental determination of the fractional speed difference as a function of , a signal-to-noise parameter expressing the ratio of the observed VMCM speed to a characteristic rms wave speed . Although significant scatter occurs, the average fractional speed difference decreases from about (at 95% confidence) at to at at and remains approximately constant at for between and 2.0. 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 , where is the true mean horizontal speed and the rms wave-induced oscillatory velocity. The combined laboratory and CODE-1 field data thus indicate that in an absolute sense, the VACM over-responds by about 18 to 20% at to % at , while the VMCM under-responds by about -5% to -6% for between 0.5 to 2.0.