Assessment of surface currents measured with high-frequency phased-array radars in two regions of complex circulation

Surface velocity data from two WERA high frequency (HF) ocean radar systems, deployed as part of the Australian Integrated Marine Observing System (IMOS), are compared with near surface currents obtained from drifters and ADCPs (acoustic Doppler current profiler). We evaluate data from two contrasting locations in the first detailed evaluation of the IMOS HF radar surface velocities. HF radar measurements are generally robust but demand quality-control procedures to eliminate obvious errors and outliers that appear temporarily or systematically in the data. A number of different quality control procedures and filters are applied and assessed including Taylor diagrams, Hampel and Savitzky-Golay filters. In addition the need for and effect of averaging are discussed. The radar measurements of surface current agreed better with the near-surface drifter currents than with the subsurface ADCP currents. Nonetheless the ADCP comparisons are consistent with those previously reported in other regions. The value of the Taylor Diagram for comparing different surface current data sets and processing approaches is demonstrated. Noise levels in the radar current spectra are used to estimate the error in the measurements and in some cases, these errors were found to approach the precision of the radar estimates. Our results give guidance on the most useful temporal sampling resolution. In particular we show that, at these sites and these operating frequencies, using 10-minute sampling without further averaging does not provide additional information because the higher frequencies are dominated by noise. Averaging the radials over 30-minutes may be sufficient for many applications.