Quad‐polarization SAR features of ocean currents

Abstract

A methodology is demonstrated to exploit the polarization sensitivity of high-resolution radar measurements to interpret and quantify upper ocean dynamics. This study particularly illustrates the potential of quad-polarization synthetic aperture radar (SAR) measurements. The analysis relies on essential characteristics of the electromagnetic scattering mechanisms and hydrodynamical principles. As the relaxation scale of centimeter-scale ocean surface scatters is typically small, radar signal anomalies associated with surface manifestations of the upper ocean dynamics on spatial scales exceeding 100 m are mostly dominated by nonresonant and nonpolarized scatters. These ''scalar'' contributions can thus efficiently trace local breaking and near-breaking areas, caused by surface current variations. Using dual copolarized measurements, the polarized Bragg-type radar scattering is isolated by considering the difference (PD) between vertically and horizontally polarized radar signals. The nonpolarized (NP) contribution associated with wave breaking is then deduced, using the measured polarization ratio (PR) between polarized signals. Considering SAR scenes depicting various surface manifestations of the upper ocean dynamics (internal waves, mesoscale surface current features, and SST front), the proposed methodology and set of decompositions (PD, PR, and NP) efficiently enable the discrimination between surface manifestation of upper ocean dynamics and wind field variability. Applied to quad-polarized SAR images, such decompositions further provide unique opportunities to more directly assess the cross-polarized (CP for HV or VH) signal sensitivity to surface roughness changes. As demonstrated, such an analysis unambiguously demonstrates and quantitatively evaluates the relative impact of breakers on cross-polarized signals under low to moderate wind conditions.

Dual-Polarized Backscatter Features of Surface Currents in the Open Ocean during Typhoon Lan (2017)

“…Kudryavtsev et al [5] indicated that the ocean surface current signatures are mostly dominated by non-Bragg backscatter through the analysis of the VV and HH polarizations from quad-polarized RADARSAT-2 SAR images. However, in this study, we remove the non-Bragg backscatter before extracting the current features.…”

“…Recently, spaceborne active microwave synthetic aperture radars (SARs) have been reported as a potential tool for the detection of sea surface currents [5]. SAR techniques can be grouped into two on October 22 are the composite data acquired for a period extending over five days.…”

On the Doppler Frequency Shifts of Radar Signals Backscattered from the Sea Surface

“…Although the mechanism of scattering from the wave-breaking zones and, accordingly, the issue of the scatterer velocity in them have not been fully clarified, much attention is paid to the development of the corresponding scattering models in [17][18][19].…”

Water surface slope spectra in nearshore and river mouth environments

“…There exists no consensus in the body of capillary wave spectral data that suggests a clear expected behavior. Observational spectra [6,9] that extend into the capillary or near-capillary regime are sparse, while empirical/parameterized spectra [13,15] vary greatly in that domain, owing largely to the diverse range of observations and theoretical frameworks invoked in their construction [1,13,15]. …”

“…These techniques rely on back-scattering of transmitted radar signals by short ocean waves. Scattering models [1] depend upon the short ocean wave spectrum (i.e., decimeters and smaller -the range for which capillary effects become significant) in the determination of the normalized radar cross section (NRCS), or σ  -a quantity of primary importance for the retrieval of meteorological and oceanographic parameters from satellite images. The ability to accurately collect these parameters remotely is a powerful one -sea surface waves are of crucial significance to all manner of interactions between the atmosphere and ocean.…”