Dual-polarized Doppler radar measurements of oceanic fronts

This paper presents high-resolution, dual-polarized Doppler radar measurements of oceanic fronts made from a shipborne platform along the northern boundary of the Gulf Stream during the second Office of Naval Research (ONR)/Naval Research Laboratory (NRL) High Resolution Remote Sensing Experiment (HIRES-II). Average radial scatterer velocity and consecutive short-time Doppler spectra are computed from the coherent backscatter data. These are compared with in-situ current records obtained with an Acoustic Doppler Current Profiler (ADCP) and visually detected whitecap events recorded with a closed-circuit video camera. For the case in which dominant waves are propagating toward the radar, the change in the across-front radial scatterer velocity for vertical polarization shows reasonable agreement with current variations observed with the ADCP. Conversely, for the case in which the radar line-of-sight direction is orthogonal to the direction of propagation of the dominant waves, the radial scatterer velocity is sensitive to changes in the wave propagation direction caused by wave-current interaction at the front. For both polarizations, the shape of the Doppler spectrum shows significant variation across fronts, particularly for horizontal polarization. These spectral variations occur coincidentally with changes in the number of visually detected whitecap events per minute obtained from the closed-circuit video recordings. These results demonstrate the potential of dual-polarized Doppler radars operating in the low grazing angle regime for remotely sensing the current variability and the occurrence of wave breaking across oceanic fronts as well as the effect of fronts on wave propagation.