Synoptic mapping of internal-wave motions and surface currents near the Lombok Strait using the Along-Track Stereo Sun Glitter technique

Abstract

A multi-component satellite remote sensing program is required to track the response of the world's oceans, lakes and rivers to climate change. Central to this endeavor is the ability to detect the motions of internal waves, swell waves and currents and hence follow energy transport and exchange. However, the present methods of monitoring the motions of water bodies from space, such as those based on altimetry or gravity measurements, are geared mostly toward applications on large spatial scales, whereas the capacity to map the fine details of hydrospheric flows is limited. This paper describes a satellite-based method of detecting wave motion and surface currents at high (in principle metric) resolution that can be applied under specific circumstances in the confined environs of narrow sea straits, lakes and rivers and that compliments the use of other high-spatial resolution techniques such as those based on Synthetic Aperture Radar (SAR). The Along-Track Stereo Sun Glitter (ATSSG) technique makes use of images of water bodies that are separated in time by roughly 1 min and are gathered in the forward-, nadir- and backward-looking directions by space-borne optical sensors performing along-track observations. When sensor viewing geometries lead to the presence of Sun glitter in these images, surface slicks, internal waves, swell waves and other phenomena become highlighted through the surface roughness changes they induce, since these in turn modulate the reflected glitter radiance. Measurement of the differential displacements between congruent sections of the surface roughness signatures present within image pairs of the stereoscopic sequence then enables internal wave or swell wave motions to be determined, while surface currents can be deduced if “passive” tracers of the flow in the form of surface roughness structures (such as slicks) are present. The application of the ATSSG technique described herein makes use of data acquired at a spatial resolution of 2.5 m by the Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) to provide the first along-track fine-scale synoptic mapping of the surface motions of individual components within a large group of internal waves and to generate supporting surface-current measurements. The PRISM data we employ were acquired to the south of the Lombok Strait (Indonesia), where highly energetic internal-wave growth takes place and where through-flow surface-current data, of the type derived by the ATSSG technique, can be of value in climate studies.