HF Radar Observation of Wave Directional Spectra in a Strong Current Regime

Dual Wellen HF Radar (WERA) systems have been observing near-surface currents and wave parameters over the Southeast Florida shelf since June 2004 as a part of the Southeast Atlantic Coastal Ocean Observing System (SEACOOS). The region of coverage includes the Florida Straits and the Florida Current (FC) which typically has maximum surface velocities approaching 2 ms"1. The echo-Doppler spectra are also routinely recorded and archived at both stations which allows post-processing to extract surface wave directional spectra using an iterative approach as implemented in Seaview Sensing reg software. Both WERA sites operated continuously during the passage of Hurricane Jeanne over the Florida Straits on 25 Sept 2004. Although it passed ^~200 km to the north of the measurement domain, the local mean winds exceeded 20 ms^-1 and rotated over 270deg. The near-surface currents during the passage of Hurricane Jeanne reflected the influence of the wind as well as the Florida Current. The effect of the wind on the near surface flow was seen in easterly and southerly flow over the shallow shelves near Florida and the Bahamas respectively as well as relatively slow flow in the center of the Florida current. Maximum current velocities were only 130 cms^-1 50-60 cms^-1 less than typical values. The interaction of these wave fields from differing directions with the high lateral shear of the western edge of the Florida Current was observed every 10 minutes.. The wind-wave component of the spectrum was observed to respond rapidly to the rotating wind-field, but effects of the horizontal shear were observed in the off-wind angle of the wind-wave peak. Tower frequencies were often observed at large angles to the local wind.