L-Band Analysis of the Effects of Oil Slicks on Sea Wave Characteristics

The purpose of this study was to investigate the characteristics of sea surface waves as they pass through oil slicks. The parameterized first-guess spectrum method (PFSM) theory-based wave retrieval algorithm was applied to 20 images of horizontal-horizontal (HH) polarization obtained using the phased-array L-band synthetic aperture radar (SAR) (PALSAR) on the Advanced Land Observing Satellite (ALOS-1). The images were collocated with simulations from the WAVEWATCH-III (WW3) model in a 0.1° grid using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-5) winds data as the forcing field. The validation of the model-simulated significant wave height (SWH) against the measurements from the Jason-2 altimeter produced a 0.66 m root mean square error (RMSE) for the SWH, with a coefficient (COR) 0.74. In this sense, the WW3-simulated waves were reliable for our work. A comparison between the SAR retrieval results and the WW3 simulations was performed using the dataset for the regions without oil slicks, which produced a 0.34 m RMSE for the SWH, with a COR of 0.79, which is less than a the RMSE of 0.52 m and the COR of 0.70 for the regions with oil slicks. Moreover, it was found that the SAR-derived SWHs were significantly underestimated by about 0.2 m in the areas with oil slicks. This difference is probably due to the underestimation of the SAR-derived wind speeds at moderate wind speeds (i.e., at wind speeds of greater than 5 ms −1 ). An additional analysis compared the SAR-derived wave spectra with those from the WW3 model as waves passed through the oil slicks. The interesting finding is that the wave energy at short wave lengths (about 30 m) is reduced by the oil slicks, causing the movement of the dominant wave spectrum to shift to longer wave lengths (about 80 m).