The dynamics of freshwater plumes at the Dutch coast as observed by high-resolution HF radar

Ground-based HF (High Frequency) radars are able to measure ocean surface currents (by means of first-order Bragg scattering) and wave spectra (by means of second-order scattering). A carrier frequency of some 30 MHz allows the mapping of currents within an area of up to 50×50 km. Recently, the University of Hamburg developed a new HF radar system, called WERA (WEllen RAdar), replacing the CODAR (COastal raDAR). One advantage of WERA is the flexiblity in range resolution between 0.3 and 1.2 km, instead of the fixed resolution of some 2 km of CODAR. This is achieved by making use of FMCW (Frequency-Modulated Continuous Wave) techniques. When operated with a linear array of receive antennas, WERA allows the mapping of surface-wave spectra within a reduced area of about 35×35 km. This paper reports on current measurements off the Dutch coast in fall 1996. Fresh water lenses could be tracked, which originate from the Rhine and Ijssel outflow and propagate along the coast. The WERA high resolution mode reveals much more details of the plume structure than attainable with CODAR. Surface current shear of up to 50 cm/s within 300 m has been observed