FY 1995

Correcting moored ADCP data for fish-bias errors at 0°, 110°W and 0°, 140°W from 1990 to 1993

Plimpton, P.E., H.P. Freitag, and M.J. McPhaden

NOAA Tech. Memo. ERL PMEL-107, NTIS: PB95-219366, 49 pp (1995)

This report describes the processing techniques utilized in producing daily averaged velocity profiles from downward-looking RDI Acoustic Doppler Current Profilers (ADCPs) mounted on surface moorings at 0°, 110°W and 0°, 140°W. The data presented are from 1991 to 1993 at 110°W and 1990 to 1993 at 140°W. Initial post-processing corrections include bin depth adjustments using historical sound velocity profiles, and current velocity adjustments using in situ temperature to estimate surface sound velocity. Of greater significance are the velocity corrections required due to the presence of pelagic fish in the vicinity of the moorings. The ADCP velocities are compared with mechanical current meters (MCMs) set at specific depths in the mooring line. This comparison indicates that the ADCP speeds are, at times, biased towards lower values due to the reflection of acoustic energy from fish. After computing a mean ADCP-MCM offset for each deployment, an empirical orthogonal function (EOF) analysis is applied to the time-varying ADCP-MCM differences. For the mean and each eigenmode, differences computed at the MCM depths are interpolated to the finer vertical resolution of the ADCP depth grid. The vertically interpolated amplitudes from the mean and the first three eigenmodes are used to produce a correction for the ADCP speeds. The ADCP current directions do not appear affected by the presence of fish and are used, along with the corrected ADCP speeds, to produce a significantly more accurate data base of daily averaged zonal and meridional velocity profiles.