HF radar observation of nearshore winds

Following Ekman’s 1905 mathematical description of the influence of wind stress on the ocean, measurement of the wind field itself has been a critical part of any effort to understand the movement of ocean currents. This is particularly true in the coastal zone, where winds and waves interact with the coastal boundary to drive spatially and temporally complex currents. In addition to understanding and predicting coastal flows, observations of near shore surface winds are fundamental to fulfilling both scientific (e.g. circulation, mixing, biological productivity, larval transport) and societal needs (e.g. shipping, wind power). Near shore wind observations, either in situ or remote, typically have high temporal resolution, or high spatial resolution, but not both. Buoy-based observations, such as those accessible via the National Data Buoy Center (NDBC), provide time series of wind products from most coastal areas but lack the spatially relevant resolution for observing many – if not most – of the critical small scale circulation processes. Recent developments have improved satellite scatterometer capabilities to within 10-15 km from shore (e.g.[2, 3]), and similarly, Synthetic Aperture Radar (SAR) achieves sub-km resolution (0.5-1 km) up to 1-3 km from shore with RMS errors of 1.4-1.8 m s−1 [4, 5]. While planned future missions such as the Waves and Currents Mission (WACM) [6] would further advance these techniques and provide maps of winds with high spatial resolution, satellite-based observations sample at periods of 12 hours or greater, limiting their scientific utility. In contrast, land-based HF radar systems routinely provide both high spatial and high temporal resolution observations of surface currents in the coastal ocean in all weather conditions. The fundamental signal observed by the radar system results from the presence of relatively short ocean waves that respond quickly to changes in wind speed and direction. In the near shore, wind observations from these systems would fill an important niche between satellite observations, which encounter difficulties close to land masses, and moored observations, for which spatially dense deployments are cost prohibitive. The spatial and temporal coverage possible, with time scales of 10s of minutes and spatial scales of 2-6 kilometers, matches the most likely resolution needed to advance the present understand and modelling of coastal ocean circulation [7, 8, 9].