Evaluation of two algorithms for a network of coastal HF radars in the Mid-Atlantic Bight

The National High Frequency (HF) Surface Cur- rent Mapping Radar Network is being developed as a back- bone system within the U.S. Integrated Ocean Observing System. This paper focuses on the application of HF radar- derived surface current maps to U.S. Coast Guard Search and Rescue operations along the Mid-Atlantic coast of the USA. In that context, we evaluated two algorithms used to combine maps of radial currents into a single map of total vector currents. In situ data provided by seven drifter deployments and four bottom-mounted current meters were used to (1) evaluate the well-established unweighted least squares (UWLS) and the more recently adapted optimal interpolation (OI) algorithms and (2) quantify the sensitivity of the OI algorithm to varying decorrelation scales and error thresholds. Results with both algorithms were shown to depend on the location within the HF radar data footprint. The comparisons nearthecenteroftheHFradarcoverageshowednosignificant difference between the two algorithms. The most significant distinction betweenthe two was seen in the drifter trajectories. With these simulations, the weighting of radial velocities by distance in the OI implementation was very effective at reduc- ing both the distance between the actual drifter and the cluster of simulatedparticles as well asthescaleofthesearch areathat encompasses them. In this study, the OI further reduced the already improved UWLS-based search areas by an additional factor of 2. The results also indicated that the OI output was relatively insensitive to the varying decorrelation scales and error thresholds tested.