Coastal ocean fronts and eddies imaged with ERS 1 synthetic aperture radar

Abstract

ERS 1 C band synthetic aperture radar (SAR) data were collected during the Norwegian Continental Shelf Experiment (NORCSEX) both in November 1991 during the ERS 1 commissioning phase and at different seasons in 1992 and 1993. Characteristic SAR image expressions are observed in relation to perturbation of the surface current‐short wave interaction across the Norwegian Coastal Current front for winds less than 10–12 m s−1. In situ measurements document the existence of alternating zones of convergence and divergence coexisting with a strong near‐surface current shear of nearly 4f (where f is the Coriolis parameter) across a distance of a few kilometers. Under calm to moderate winds, i.e., 4–7 m s−1, characteristic expressions of upper ocean circulation features also include the manifestation of eddies through the presence of surface film, which damps the Bragg waves. Comparison of a near‐coincident National Oceanic and Atmospheric Administration advanced very high resolution radiometer image and an ERS 1 SAR image supports the interpretation that surface current fronts are imaged by SAR. In combination with a SAR image simulation model, the relative quantitative importance of shear, convergence, and divergence along the front is examined. Although the model formulation is simple and the absolute magnitude of the perturbations is uncertain, the study shows that the SAR images can sometimes be used to interpret frontal dynamics, including growth and decay of meanders.

Mesoscale structures viewed by SAR and AVHRR near the Canary islands

“…However speeds reported here are typical of ship board wind measurements in the area (Barton et al, 2000). Johannessen et al (1996) had comparable results which they discuss in more detail in the light of in situ measurements of currents and atmospheric conditions. The bright lines observed in the SAR frames, that coincide with the periphery of the anticyclonic eddy, the boundary between the cyclonic eddy and the filament, and in the filament itself are all indications of enhanced surface roughness in zones of horizontal shear, as also observed by the Fu and Holt (1983) with SEASAT data in the California current.…”

Robust of Doppler Centroid for Mapping Sea Surface Current by Using Radar Satellite Data

“…In fact, the Doppler frequency estimated from SAR data overcomes from the data are sampled with the Pulse Repetition Frequency (PRF) and an ambiguity about the correct PRF band [13] . In addition, the maximum shift along azimuth direction is due to strong nonlinearity occurred between radar signal and surface orbital velocity which can be called as velocity bunching effect [1][2][3][4][5][6] . In contrast, WDAR and MLBF estimators produced a clear and sharp Doppler spectra intensity peak.…”

“…Furthermore, Chapron et al [2] have acquired a surface Doppler velocity by using an average over the random wave phases. In this context, the Doppler Centroid frequency anomaly divided by the electromagnetic wave number assuming that Doppler Centroid frequency anomaly is a simple geometrical mean weighted by normalized radar section of each element [3] . Romiser and Thompson [10] , nevertheless, stated that when Doppler Centroid estimators are applied to SAR data, biased estimates are often obtained because of anomalies in the received data.…”

Observing Surface Circulation of the Western Mediterranean Basin With Satellite Imagery

“…At moderate wind speeds (up to about 5 m s -1 ), eddies usually appear in SAR images due to the presence of natural films on the sea surface which act as passive tracers of surface currents (Johannessen et al 1996;Gade et al, 2013). Such surfactants dampen the small-scale surface roughness, thereby causing slick areas on the sea surface and reducing the radar backscatter (Alpers and Hühnerfuss, 1989).…”