COASTWATCH95 - ERS-1 2 SAR DETECTION OF NATURAL FILM ON THE OCEAN SURFACE/

Ocean mesoscale phenomena such as eddies and current convergence zones can often be seen in synthetic aperture radar (SAR) images due to cha racteristic patterns caused by natural film induced damping of the wav es. Such films have also been found to exert a significant effect on a ir-sea gas exchange, which may be important for the global scale clima te system. Satellite SAR may prove very useful to quantify the extent of natural film. To investigate the composition of these films and the ir effect on radar return, we compared samples of the sea surface with ERS 1/2 SXR images of coastal ocean areas during the COASTWATCH'95 ex periment, Simultaneous observations were made with a shipmounted C ban d dual-polarized Doppler radar, and surface drifters were deployed to investigate the surface current variations in the vicinity of differen t slicks (areas where the short surface waves sensed by a radar are da mped). One confirmed case of natural film was thus verified to be caus ed by a convergence zone. The study also showed that the films investi gated during COASTWATCH'95 were generally less concentrated and origin ated from marine organisms, compared to the films with terrestrial inf luences found in a previous experiment in a fjord [Espedal et al,, 199 6]. 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 co llected 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, wh ile already at 5-10 m/s all SAR imagery had under 5% film coverage.