Surface Current Measurements During Safe Seas 2006: Comparison and Validation of Measurements from High-Frequency Radar and the Quick Release Estuarine Buoy

Abstract

The National Oceanographic and Atmospheric Association's (NOAA) 2006 Safe Seas Oil Spill Drill, conducted just outside the Golden Gate, was a prime opportunity to test the value and effectiveness of three recently deployed 13 MHz coastal radars that are part of the Central and Northern California Ocean Observing System. These Coastal Ocean Dynamic Application Radar (CODAR) systems were deployed by the Coastal Ocean Currents Monitoring Program (COCMP) to measure surface currents up to 85 km offshore from Point Reyes to just north of Half Moon Bay. CODAR systems measure surface currents by transmitting radio waves over the ocean and use the Doppler-shifted return sea echo to extract surface current velocities. Safe Seas 2006 was the first demonstrated use of High- Frequency Radar (HFR) to assist in oil-spill response in real time. Surface current maps were posted to the web hourly during the simulated oil spill to monitor surface current structure during the 48-hour exercise. NOAA's Quick Release Estuarine Buoy (QREB) was also deployed at the location of the simulated oil spill to obtain oceanographic environmental data in real time. The QREB is equipped with an Acoustic Doppler Current Profiler (ADCP), which provided a vertical profile of currents near the location of the simulated spill. In an effort to determine the reliability of the data produced from both measurement devices during the exercise, HFR total vector data from the three coastal systems were compared to the data acquired by the QREB surface bin located at approximately 3 m depth. Also, to verify individual HFR site performance, radial data from each site were compared with their respective radial components from the QREB data. Total-vector comparison results reveal strong correlation in both the cross-shore (R2 = 0.69) and along-shore (R2 = 0.90) components with RMS differences of less than 0.09 m/s. Both instruments also observed the same dramatic shift in along-shore current direction during the two-day exercise. Radial comparisons revealed strong correlation as well for the two HFR systems that acquired data at the QREB location. Endpoints of recovered drift cards released during the exercise also qualitatively correlate with trajectories produced from the HFR current maps. Further, tidal analyses were performed on the QREB and HFR data, utilizing Pawlowicz's widely accepted T-Tide algorithm, to further validate measurements made by these instruments. Despite the short data set, these analyses showed pronounced signals of K1 and M2 constituents in good agreement between the QREB and HFR instruments. The consistent comparison results described in this paper show that HFR can add significantly to the effectiveness of surface current mapping over a large area during oil spills and can complement the QREB measurements to enhance oil-spill response.