Real-time surface current information is a valuable supplement to understanding coastal air-sea interaction and dynamical processes at the coastal scales. Coastal surface current information may be correlated to winds and tidal currents among other physical phenomena. High-frequency (HF) radars have been used for measuring surface current fields and ocean-wave spectra. The physics behind HF radar is based on backscattering from a moving rough sea surface. The Radar transmits electromagnetic waves of 6-30 MHz (50-10 m wavelength), which travel along the sea surface beyond the horizon by ground wave propagation and are scattered back from ocean waves of half the electromagnetic wavelength (Bragg scattering). The scattered signals are measures of the Doppler spectrum caused by moving waves and speed of the surface currents carrying the ocean waves. Guided propagation along the conductive sea surface (ground wave) allows measurements beyond the horizon. It can also be inferred Ocean wave height and the wave directional spectrum using second-order sea echos of the Doppler spectrum. The Doppler shift of the backscattered signal is used for measuring the radial current speed relative to the radar site. If the two radar sites measure the radial velocity of a patch of water from two different angles, it is possible to calculate the two horizontal components of the surface velocity. The surface current measured is a horizontal mean over several km in both range and azimuth, over approximately the upper 0.5-1.0 m of the ocean (penetration depth of scattering ocean waves), and over some 10 min measuring time. These radar sites provide coastal-ocean surface current and wave information offshore out to 300 km. More detailed descriptions of the theory of HF radar can be found in numerous articles (e.g., Gurgel et al. 1999; Barrick et al. 1985).
As part of the integrated ocean observing system (IOOS), the US has installed a number of HF radars on the west and east coasts of US. Prototype real-time data architecture, initially developed through funding from the National Science Foundation (NSF), is now being integrated by the Coastal Observing Research and Development Center (CORDC) at the Scripps Institution of Oceanography with existing HF radar data networks through a joint development program administered and managed by the National Data Buoy Center (NDBC) and the National Ocean Service (NOS), with oversight provided by the National Oceanic and Atmospheric Administration's (NOAA) IOOS program office (Terrill et al. 2006). An excellent online reference containing an introduction to the principles of HF Radar can be found on the Rutgers University Coastal Ocean Observation Lab (RUCOOL, http:// marine.rutgers.edu/cool). The coastal radar locations on the east/west coast of the US and daily average values of surface currents (6 km) derived from HF Radar along the coast of the US are available at http://cordc.ucsd.edu/projects/mapping/maps/.
The validation of both wave with moorings and current observations with surface drifters are explained in detail by Kohut et al. (2008). Surface current observations using HF Radar and its assimilation into the NewYork Harbour observing and prediction system has been reported by Gopalakrishnan (2008). There are many coastal ocean radars that have been installed all around the coastal stations. The data provided by the coastal ocean Radar is already useful for many operational applications and Research use (http://www.codar.com/bib_05-present.htm).
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