Surface Wave Induced Skew Errors In Acoustic Doppler Current Profiler Measurements From High Shear Regimes

Abstruct-Downward-looking RD Instruments acoustic Doppler current profilers (ADCPs) are presently mounted on surface moorings along the Pacific equator as part of the Tropical Ocean-Global Atmosphere Program. In the equatorial region, the South Equatorial Current/Equatorial Undercurrent system creates high vertical shear in the zonal velocity. ADCP skew error is known to occur in high vertical shear regimes due to the misposition of the ADCP tracking filter with respect to the frequency of backscattered sound energy. Skew errors in the moored ADCP current measurements from the eastern equatorial Pacific are particularly troublesome in the first bin below the surface. The filter position for the first bin is set by the values measured from previous pings according to the E-command setup. Calculations of the expected skew error from typical mean zonal shears in the equatorial region were made prior to deployment of the moored array using equations provided by RD Instruments. However, the skews calculated from the mean curve only account for about 1/3 of the velocity errors observed in a moored ADCP system at O", llOoW in spring 1991. This underestimate of error is due to the fact that the skew must be computed on a ping-to-ping basis, incorporating the vertical and horizontal velocity changes that occur each ping. A model for surface wave induced skew error i s presented which includes ping-to-ping velocity changes in the skew calculation. In high shear regimes, such as the equatorial region, the model calculates skew errors of equal magnitude to those observed in the data. Alternative ADCP setup commands, tested with shipboard data, are presented to minimize tracking filter skew errors in future measurements.