Fall 1984 Delaware Bay acoustic doppler profiler intercomparison experiment

Abstract

A Truth intercomparison experiment was conducted near an RD Instruments (RD) acoustic Doppler current profiler (ADCP) in the vicinity of Brandywine Shoals lighthouse (MLLW depth of about 10.7 m) during the period 19 September through 16 October 1984. This intercomparison was the first by NOAA involving the RD, a 4-beam, 15\deg Janus configuration incoherent Doppler sonar operating at 1.2 MHz with 1.1-m depth bins. Intercomparison instrumentation consisted of two NBIS Smart Acoustic Current Meters (SACMs), two EG&G Sea Link Vector Measuring Current Meters (VMCMs), one Marsh-McBirney, Inc., Model 585 electromagnetic current meter (MMI), three Grundy current meters (GRs), and three Aanderaa Recording Current Meters (RCMs). One of the RCMs was equipped with the new-style shielded, paddle rotor instead of the traditional Savonius rotor. Water level and meteorological data were also recorded. There were four depth levels at which intercomparison was possible: from about 3.3 m below MLLW (level I) to about 7.8 m (level IV). Currents at the intercomparison site were dominated by the semidiurnal tide, with peak speeds of about 2 kt (100 cm/sec), and were strongly polarized in the along-channel direction. The experiment yielded generally good agreement between instruments, and all the records were highly coherent in the along-channel component. Nevertheless, significant and consistent differences between instrument types were found, particularly in speed and direction of the 15-minute averages. In the near-surface zone, the occurrence of large, physically unrealistic, current shear in the upper RD bins was strongly correlated with water level and with deviation of the backscatter spectral bandwidth from its nominal 700 Hz level. The generally good results obtained with the RD suggest that it is suitable for NOAA's use in operational tidal current circulation studies, and, in fact, will result in improved accuracy in these studies relative to older instruments (RCM and GR).