Tracking of fresh-water plumes in Dutch coastal waters by means of HF radar

Progress in electronics and computer techniques allowed the design of a new high-frequency (HF) radar system at the University of Hamburg (Germany). WERA (Wellen Radar) is a frequency-modulated continuous-wave (FMCW) radar which, when linked to a linear array of receive antennae, simultaneously measures surface currents and ocean waves. Operating at 27 MHz, the highest spatial resolution is 0.3 km. The area covered in this mode is about 40 km×40 km. Results from an experiment at the Dutch coast in boreal fall 1996 are presented. Occasionally, a high-velocity jet has been observed which travels northwards along the coast. At the edge of the jet, some 15 km to 30 km off the coast, the high-resolution mode of WERA revealed current shears of up to 0.002 s ^-1. It is assumed that the jet carries fresh water plumes which originate from the Rhine and Ijssel outflow into the North Sea. In addition to the surface currents, the spatial distribution of backscatter strength is investigated. Some evidence is found that, during phases of low wind, fresh water plumes become visible as areas of reduced backscatter. This finding is in accordance with the dependence of HF ground-wave attenuation on sea-water conductivity