Evaluating radial current measurements from CODAR high-frequency radars with moored current meters

The performance of a network of five CODAR ( Coastal Ocean Dynamics Application Radar) SeaSonde high-frequency (HF) radars, broadcasting near 13 MHz and using the Multiple Signal Classification ( MUSIC) algorithm for direction finding, is described based on comparisons with an array of nine moorings in the Santa Barbara Channel and Santa Maria basin deployed between June 1997 and November 1999. Eight of the moorings carried vector-measuring current meters (VMCMs), the ninth had an upward-looking ADCP. Coverage areas of the HF radars and moorings included diverse flow and sea-state regimes. Measurement depths were similar to1 m for the HF radars, 5 m for the VMCMs, and 3.2 m for the ADCP bin nearest to the surface. Comparison of radial current components from 18 HF radar-mooring pairs yielded rms speed differences of 7-19 cm s(-1) and correlation coefficients squared (r(2)) in the range of 0.39-0.77. Spectral analysis showed significant coherence for frequencies below 0.1 cph ( periods longer than 10 h). At higher frequencies no significant coherence was found, and noise levels corresponding to 6 cm s(-1) rms were evident in the radar data. Errors in the radar bearing determination were found in 10 out of 18 comparisons, with a typical magnitude of 58-108, and a maximum of 19degrees. The effects of bearing errors on total vector currents were evaluated using a simple flow field and measured bearing errors, showing up to 15% errors in computed flow speeds, and up to; 98 errors in flow directions.