NOAA’s recent field testing of current and wave measurement systems – part i

The National Oceanic and Atmospheric Administration's (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) currently maintains twenty-two operational Physical Oceanographic Real- Time System (PORTS) observatories throughout the United States' coastal regions. Ocean current observations are among the multiple PORTS data products that provide critical support for safe navigation and a variety of scientific and engineering applications. To ensure that its observing network provides the most accurate and up-to-date products available, CO-OPS routinely conducts test and evaluation of newly available oceanographic sensors and measurement systems. From October 10 - December 3, 2014, CO-OPS conducted a field test in the South Chesapeake Bay which included three acoustic current profiling and wave sensors deployed in bottom mounted, upward looking configuration 1) the TRDI 500 kHz Sentinel V, 2) the Nortek 600 kHz AWAC-AST, and 3) the Nortek 500 kHz Signature 500 AD2CP; and an AXYS Technologies TriAXYS
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directional wave buoy with current measurement capability provided by an integrated Nortek 600 kHz Aquadopp acoustic sensor. Also included in the deployment was a bottom mounted TRDI 600 kHz Workhorse Sentinel acoustic current profiling sensor. The test site was near the mouth of the bay where currents are predominantly tidal and the average water depth is approximately 15 meters (50 feet). During the test, currents at the site ranged in magnitude from approximately 0-0.75 m/s on a daily basis and reached as high as 1.2 m/s (2.3 knots) on one occasion. Also, three high wind storm events passed through the region and significant wave heights exceeded 2 m on multiple occasions. Data results presented in this paper focus on comparison of currents measured by two of the bottom mounted sensors, the TRDI Workhorse and Nortek AWAC, and the TriAXYS
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wave buoy's Nortek Aquadopp sensor. Currents measured by the bottom mounted AWAC and Workhorse sensors showed excellent agreement throughout the entire test; differences in principle current axis headings remained less than 5 degrees and root mean squared deviations (RMSD) between the measured current magnitudes were within 4-5 cm/s over all profile depths. Comparisons of currents measured by these two bottom mounted sensors and the TriAXYS
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buoy's Aquadopp sensor showed good agreement in current direction but comparisons between current magnitudes varied throughout the test. During the first 20 days, currents measured by bottom mounted sensors did not exceed 1 m/s and comparison between bottom versus buoy mounted sensors' magnitudes were excellent; RMSDs remained within 5-6 cm/s. However, over the final 34 days of the test when currents measured by bottom mounted sensors often exceeded 1 m/s, the bottom versus buoy mounted sensors' current magnitude RMSDs were approximately twice as large, in the 11-14 cm/s range. Preliminary results provide no clear indication of significant effect of wave conditions on bottom versus buoy mounted ADCP comparisons. Rather there is a significant correlation between bottom versus buoy mounted sensors differences and the bottom mounted sensors' measured currents; the buoy mounted Aquadopp sensor appears to be biased toward higher values the majority of the times when bottom mounted sensors' measured currents exceeded 1 m/s. Several key features of current data collected during this test will be presented along with initial results from the investigation into differences between bottom versus buoy mounted ADCPs' measurements.