ULTRA-WIDE-BAND RADAR STUDIES OF STEEP CRESTED WAVES WITH SCANNING LASER MEASUREMENTS OF WAVE SLOPE PROFILES

We report results of ultra wide-band radar sea spike experiments using steep and weakly breaking non-linear water surface features in a wave tank. To generate these features we used a 1 s paddle wave and wind w aves for a sequence of wind speeds. A scanning laser was used to measu re synchronously the surface slope profile across 12 cm along the wave propagation direction once per radar pulse. A time domain reflectomet er (TDR) radar transmitted short horizontally polarized pulses at X-ba nd, several hundred picoseconds long, to give a range resolution of 10 cm. A radar range of 36 cm was digitally sampled so that surface feat ure echoes could be tracked through the area continuously with 5 ms te mporal resolution with each instrument. We report results considering the wave slope component in the propagation direction and the correspo nding curvature component. For the conditions studied, two types of fe atures which produce sea spike radar echoes were generated-a non-linea r feature near the crest front of the wind wave, caused by extreme ste epening as a result of the passage of the paddle wave, and a steepened blocked wind wave in the trough of the paddle wave, caused by the loc al orbital current of the 1 s wave being nearly equal to and opposite the phase velocity of the wind wave.