COASTAL LEE WAVES ON ERS-1 SAR IMAGES

Alternative dark-bright patterns on ERS-1 synthetic aperture radar (SA R) images of the west side of the Taiwan Strait taken on December 8, 1 994, were recognized to be the sea surface signature of a coastal lee wave. Such waves are called coastal lee waves because they occur along the lee side of the coast. The coastal lee waves appeared in the form of a wave packet distributed within an offshore band 20-40 km wide. T he first packet, which occurred in the northern portion of the observe d area, contained six waves with variable wavelengths (defined as the spatial separation between two waves) from 1.7 to 2.7 km. The second p acket, in the middle, contained 10 waves with a relatively uniform wav elength of 4.2 km. The third packet, in the southern portion, containe d 17 waves with an average wavelength of 2.0 km. The crest lengths wer e from 20 to 80 km. Local meteorologic parameters observed simultaneou sly at Fuzhou, China, close to the imaged area, showed an offshore win d of 1.5-3.5 m/s and at land surface air temperature of 19 degrees C, which was 4 degrees C lower than the sea surface temperature (SST). Th us the lower atmospheric boundary conditions at imaging time were very favorable both for generating the land breeze circulation and small w ind waves on the sea surface, which are in the Bragg-scattering wavele ngth band of the C band ERS-1 SAR. A physical model of a three-layer a tmosphere was developed in order to explain how the land breeze circul ation can generate the coastal lee waves. The results showed that the vertical velocity disturbance caused by the wind convergence at the la nd breeze frontal zone is of vital importance for the generation of co astal lee waves, and the model gave very good estimates of the process es observed. The SAR imaging mechanisms of the waves were analyzed in detail. The differences between coastal lee waves and ocean internal w aves, which appear as similar alternative dark-bright patterns on SAR images, were also discussed.