Comparison of BASS and VACM current measurements during STRESS

Current measurements from a vector-averaging current meter (VACM) on a subsurface mooring and a benthic acoustic stress sensor (BASS) on a bottom tripod are compared to assess their relative accuracy. The instruments were deployed off northern California at a midshelf site (water depth approximately 90 m) as part of the STRESS (Sediment Transport Events on Shelves and Slopes) field program. The subsurface mooring and bottom tripod were within a few hundred meters of each other, with the BASS 5.0 m and the VACM 6.7 m above the bottom, during two tripod deployments of 49 and 32 days in the winter of 1988/89. Speed differences between the VACM and BASS current observations have a mean of 0.2 cm s⁻¹ and a standard deviation of 1.2 cm s⁻¹. If the mean speed profile is logarithmic, the expected mean speed difference due to the vertical separation is about 0.4 cm s⁻¹. The average speed difference between the VACM and BASS increases as near-bottom wave orbital velocities get large relative to hourly averaged currents, consistent with laboratory studies of VACMs in oscillating flows. Direction differences have a mean of 1° and standard deviations of about 5° for speeds greater than 10 cm s⁻¹. The relative accuracy of the corresponding velocity measurements is ±2 cm s⁻¹ (mean differences less than 0.6 cm s⁻¹ and standard deviations of about 1 cm s⁻¹).

The equations used to convert VACM rotor rotation rates to current speed we based on a calibration study by Woodward and Appell rather than one based on a study by Cherriman that is routinely used at the Woods Hole Oceanographic Institution. The former yields closer agreement between the BASS and VACM speed measurements during STRESS (mean speed difference 0.2 cm s⁻¹ versus 1.4 cm s⁻¹).