COASTWATCH'95: ERS 1/2 SAR detection of natural film on the ocean surface

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Abstract
Ocean mesoscale phenomena such as eddies and current convergence zones can often be seen in synthetic aperture radar (SAR) images due to characteristic patterns caused by natural film induced damping of the waves. Such films have also been found to exert a significant effect on air‐sea gas exchange, which may be important for the global scale climate system. Satellite SAR may prove very useful to quantify the extent of natural film. To investigate the composition of these films and their effect on radar return, we compared samples of the sea surface with ERS 1/2 SAR images of coastal ocean areas during the COASTWATCH'95 experiment. Simultaneous observations were made with a shipmounted C band dual‐polarized Doppler radar, and surface drifters were deployed to investigate the surface current variations in the vicinity of different slicks (areas where the short surface waves sensed by a radar are damped). One confirmed case of natural film was thus verified to be caused by a convergence zone. The study also showed that the films investigated during COASTWATCH'95 were generally less concentrated and originated from marine organisms, compared to the films with terrestrial influences found in a previous experiment in a fjord [Espedal et al., 1996]. The dependence of the existence of the films on wind speed is also investigated, and an estimate of natural film distribution during the experiment period is given, using a total of 71 ERS 1/2 SAR images collected over the same coastal area under a variety of wind conditions. Up to 40% natural film coverage was found for 2.5 m/s wind speeds, while already at 5–10 m/s all SAR imagery had under 5% film coverage.