Detection of Current-rips (shiome) Using PALSAR/PALSAR-2

Abstract

The Phased-Array L-band Synthetic Aperture Radar-2 (PALSAR-2) high-pass-filtered image is compared with the Moderate Resolution Imaging Spectroradiometer (MODIS) sea surface temperature (SST) and Chlrolophyll-a (Chl-a) images. The comparison shows that the positions of line-shaped bright (ridge) patterns in the image correspond with large SST gradients, i.e., SST fronts. Moreover, the comparison with the Chl-a image represents some local Chl-a maxima along the SAR ridge patterns in the PALSAR-2 contrast image. In order to examine the relationship between the SAR line-shaped bright patterns and surface currents comprehensively, the time series of the PALSAR images are compared with the High-Frequency (HF) radar surface currents. The positions and strengths of the SAR line-shaped bright patterns generally correspond with those of current shear, suggesting a general theory that convergence areas induced by large current shear are imaged bright through the modulation of ocean surface roughness. The SAR-derived line-shaped bright pattern thus indicates current-rips (Shiome) characterized physically as the convergence of surface currents. The effect of background wind fields on the SAR line-shaped bright patterns is also investigated using the SAR-derived wind fields. It is found that the SAR line-shaped patterns are not identified under strong winds stronger than 10m/s even if the current shear is large.