THE USE OF A COASTAL HF RADAR SYSTEM FOR DETERMINING THE VECTOR FIELD OF SURFACE CURRENTS

The HF coastal radar, using backscatter from sea waves of the electromagnetic waves emitted from land-based coastal stations, allows mapping sea surface currents along the coast with a range of several km. This paper describes the use of the HF radar system installed, within the framework of the MURST PRISMA 2 Project, to measure the surface current field in the near shore area in front of the Ancona’s coast (Northern Adriatic Sea). The system has been operating continuously since September 1997 to the spring 2000. Its use is fundamental because of the possibility of monitoring a wide coastal area, that otherwise would be difficult to cover, demonstrating its capability of being a useful supplement in data collection with standard oceanographic field measurements. Preliminary results show the well-known southeastward (i.e. alongshore) coastal jet that extends its influence as far as 10-15 Km from the coast; farther offshore a cyclonic eddy, with a diameter of 10-20 km typically occurs. This circulation can be occasionally reversed showing an anticlyclonic eddy that forces a northward coastal current with a far less energetic signature. The spectral analysis reveals a propagation of the semidiurnal frequencies from NW to SE according to the propagation of a Kelvin wave theory; conversely an exponential increase from the coast to offshore has been identified for the near-inertial frequencies. An analysis of the mean kinetic energy during a Bora wind episode describes the typical dynamic of a surface layer driven by an impulsive atmospheric forcing.