Improved quality of National Data Buoy Center (NDBC) Acoustic Doppler Current Profiler (ADCP) measurements

In 2006, NOAA´s National Data Buoy Center (NDBC) began an effort to add ocean sensors (directional waves, surface currents, current profiles, and ocean temperature and salinity) to its fleet of weather buoys. In this paper, we report on the improvements in the quality of ocean currents collected from the Acoustic Doppler Current Profiler (ADCP). Initially, the ADCP was deployed in a cage that was suspended below the bridle of the buoy. It was an effort to remove the motion of the surface buoy from the current record. Measurements were made at intervals of 30 to 45 seconds over an hour´s time and averaged. The cage required a lengthy cable to carry the current information to the payload within the buoy. The movement (twisting, swinging, and upand-down motion) of the cage put stress on the cable, which was also exposed to long line gear and other fishing tackle, and the failure rate was high. Several iterations of cable were attempted, but the failure rate of the real-time currents remained high. Because of the averaging method, the currents were under-sampled and represented a considerable period of the changing tidal current. The ADCP was moved inside the bridle just below the buoy to shorten the exposure of the cable to the environment. An additional result of this method is to sample closer to the ocean surface. The sampling interval was increased to 0.5 or 1.0 Hz and the samples are averaged over five minutes. The resulting measurements represent a more appropriate instantaneous current speed and direction and average out the motion of the buoy. A comparison of identically configured buoyand bottom-mounted ADCPs at Station 41036 off the North Carolina Coast indicate that the methodology produces statistically equivalent currents at all but the surface and bottom bins [1]. ADCP data from a large number of oil and gas platforms in the northern Gulf of Mexico are collected, quality controlled, and disseminated to the public by NDBC. The quality control algorithms- were developed by a consortium of oil industry, ADCP vendor, and Minerals Management Service experts and implemented by NDBC to quality control the realtime ADCP data in March 2006 [2]. The algorithms were developed for low frequency (38 and 75 kHz) Teledyne RD Instruments (TRDI) ADCPs. The algorithms have been expanded to include the 300, 600, and 1200 kHz TRDI ADCPs. The algorithms test for echo amplitude, percent good beams, error velocity, and horizontal velocities. Additionally, a test to determine the presence of the surface or bottom allows these bins to be included in the data stream as the tide changes. These algorithms have been implemented into NDBC´s realtime processing stream.