Validation of Surface Current Measurements in the Northern Adriatic Sea from High-Frequency Radars

The performances of a shore-based high-frequency (HF) radar network deployed along the coast of the Venice lagoon (northern Adriatic Sea) are discussed based on a comparison with a single bottom-mounted ADCP deployed in the shallow-water area offshore of the lagoon for a 40-day period in August-September 2005. The analyses, carried out using currents representative of the first meter for the HF radars and 2.5 m for the ADCP, gave rms differences of radial currents in the range of 8.7-14.7 cm s(-1) (correlation 0.37-0.82) for the ideal pattern and 8.4-20.5 cm s(-1) (correlation 0.14-0.84) for the measured pattern. Good correlation was found between surface current vectors and moored data (scalar correlation up to R 5 0.83, vector correlation rho 5 0.78, veering angle 6 degrees). Comparison metrics were improved for the low-passed currents. Angular offsets ranged between +6 degrees and +11 degrees. Differences depended primarily on the geophysical variability within the water column. Bearing offsets also contributed because they lead to comparisons with radial velocities at erroneous angular sectors. Radar performances were severely affected by strong northeasterly wind pulses in their early stages. An increased broadband noise, spread over the entire Doppler spectrum across all ranges to the antennas, masked the Bragg peaks and determined the loss in radar coverage, introducing gross underestimations of both radial velocities and total currents.