Measuring waves with a compact HF radar

Ocean wave conditions impact many ways in which humans interact with the ocean, from the safety of recreation at the beach to the viability of offshore operations. Wave conditions are also topical from a research perspective, controlling processes such as coastal erosion and ocean mixing. Therefore, being able to characterize wave conditions on broad spatiotemporal scales is extremely valuable. A network of High-Frequency (HF) radar systems can provide measurements of wave conditions in near-real time along the coast where better observations are needed. Measurement of wave parameters such as significant wave height, wave period and wave direction is a secondary function of the SeaSonde HF radar. Waves are measured with SeaSonde HF radars from the second-order portion of the echo spectrum. The Doppler shift of the radio transmission from the SeaSonde contains information about the orbital velocity of the primary Bragg waves and the larger waves that they ride on. Since the wave data is dependent upon the occurrence of both Bragg and larger surface gravity waves, there is a minimum threshold for sea states in which reliable wave parameters can be determined. There is also a limiting factor for the radar in large sea states as the first-order spectra merge with the second-order and interpretation of the spectra becomes impossible with existing methods. We have tested methods for wave extraction and will present the results here. Our analysis explores the frequency-dependent threshold wave conditions for reliable wave parameter measurements, and which systems provide the best measurements. We also tested different radio waveform parameters to see which performed best in different environmental conditions. The study focuses on in-situ wave measurements from National Data Buoy Center (NOAA) buoys within the domain of the HF radar network, deployed offshore of Long Island and Delaware Bay, as well as mooring deployments of opportunity closer to the coast. Measurements will also be evaluated within the context of larger scale wave models routinely run in the area by NOAA. The threshold conditions determined by this study will guide the application of HF radar-based wave estimates to surf zone conditions by local weather forecast offices.