Space-time properties of radar sea spikes and their relation to wind and wave conditions

Low-grazing angle (LGA) radar sea spikes were observed with a high-resolution, dual-polarization, X band imaging radar deployed on the floating instrument platform (FLIP) during the Marine Boundary Layer Experiment (MBLEX) held off the California coast during April-May 1995. Spatiotemporal statistics of observed sea spike events are presented, including duration, velocity, and directional distributions. The dependence of these parameters on wind and wave conditions is compared to theoretical predictions and to similar measurements of breaking wave signatures obtained with passive acoustic techniques by Ding and Farmer [1994]. The density of sea spikes (events per unit time per unit area) and the fractional surface coverage are estimated as a function of friction velocity and compared to theoretical predictions. Though we find consistency between the dynamical aspects of sea spike events and of acoustic wave-breaking signatures, we do not observe a predicted cubic u* dependence of sea spike density. Differences may be a consequence of the specific nature of low-grazing angle scattering at X band land higher) frequencies. We observe an approximately quadratic dependence on u* of sea spike fractional surface coverage which is insensitive to the choice of backscattered power threshold over a 9 dB range.