The Interaction of Crude Oil on the Sea Surface with Ocean Fronts observed by SAR

Kurzfassung

Changes in the sea surface roughness are usually associated with a change in the sea surface wind field, an effect that has been exploited to measure the sea surface wind speed (often tuned to 10 meter height as u10) by scat-terometry. Features on the sea surface associated with change in roughness are, e.g., Lee waves, wind rolls due to atmos-phericrolls, wind shadows, swell waves and oceanographic effects due to upwelling, Langmuir cells, current fronts and wakes as well as natural and man-made slicks. Such features can be observed on synthetic aperture radar (SAR), because of the change in Bragg backscatter of the radar signal by damping of the resonant ocean capillary waves. Further this change in sea surface roughness can as well be observed in the sun glint area of op-tical imagery. Fine scale features like oceanographic fronts, turbulent currents, eddies and small slicks of natural origin or oil seep-ages are best observed on high resolution imagery, that is now freely available to the scientific community. Especial-ly using new radar frequencies like the L-band or the high resolution from X-band satellites as well as new modes of polarization, sea surface features have been analyzed in several campaigns, bringing very different datasets together thus allowing for new insight in small scale processes at a larger area. Oil seepages and spills change the roughness of the sea surface and can thus be easily detected by synthetic ape ture radar (SAR), causing dark areas on the satellite images due to reduced backscatter. Oil spills cause sheen and a change of colour on optical imagery such as observed on Sentinel-2 data. During a field experiment in the Gulf of Mexico at the estuary of the Mississippi river in April 2017, we collected a variety of different SAR and optical datasets both showing persistent oil seeps south of the Mississippi delta. We investigated oil spill imag-ing under different environmental conditions as well as for different radar wave lengths and polarizations togeth-er with optical and in situ data. We acquired radar data from the two C-band satellites, RADARSAT-2 and Sentinel-1, as well as TerraSAR-X band imagery and compared the detected oil to in situ data as part of the GoMRI Campaign. Sumesoscale Pro-cesses and Lagrangian Analysis on the Shelf (SPLASH) research cruise. During the cruise oil was detected visu-ally directly at the surface and described by the NOAA scale. It could be shown that the size of visually detecta-ble oil is much smaller than the dark areas caused by the smoothing of the sea surface by an extended monomo-