Abstract

The satellite-based Synthetic Aperture Radar (SAR) is a high resolution sensor which provides synoptic views of several oceanic processes such as surface and internal gravity waves, density fronts, and shallow water bathymetry. This type of sensor is particularly useful in coastal areas where high resolution is required. However, archived datasets of satellite-based SAR imagery are severely limited by the long gaps between consecutive images which do not allow traditional time series analysis. In order to overcome some of the temporal limitation of available datasets, a method for tidal analysis of SAR images was proposed. The method is based on the assumption that, in coastal areas tides are the strongest and most repetitive forcing known to affect ocean processes detectable by radars. Using RADARSAT-1 SAR images of the mouth of Delaware Bay and harmonic analysis parameters, the M2 tidal modulation of density fronts and bottom topography was studied. The M2 tidal variability of frontal signatures agreed well with previous results based on high resolution in-situ sampling. The M2 tidal signatures associated with two prominent topographical features on the mouth of the Delaware Bay were analyzed separately. The radar signature which has been previously interpreted as a counter-intuitive jet is shown here to be associated with the Hen and Chickens Shoal. The radar signature of the Prissy Wicks Shoals presented the clearest M2 modulation but it is affected by local enhanced wave-breaking independently detected in high resolution aerial photography. The method developed in this studied was compared to ideal and realistic numerical simulations of radar imaging of the Prissy Wicks Shoal. In the ideal scenario, in which the wind and radar parameters were kept constant, the predicted backscattering across the shoals showed a complete M2 tidal oscillation. In the realistic scenario only one parameter was kept constant while others were allowed to vary. When incidence angle was kept constant the M2 oscillation in the backscattering signal was not predicted during the entire tidal period. Similarly it was not possible to observe the M2 fluctuation using RADARSAT-1 data. However, when wind direction was kept constant the M2 oscillation was predicted for part of the tidal period. A M2 oscillation was also observed in RADARSAT-1 data but for an even shorter duration. Based on the radar imaging simulations it is suggested that the M2 backscattering oscillations cannot be clearly observed in RADARSAT-1 SAR image because of a combined effect of speckle, variable wind direction, and other processes not included in the model, such as wave-breaking fronts. Continuous airborne radar surveys could provide more insights into this apparent limitation. It would be particularly important to choose areas with an even number of crosswind/upwind conditions, thus providing more information on the directionality aspects of radar imaging of wave-current interaction. This research showed some of the limitations that need to be overcome before analyzing archived datasets of satellite-based SAR imagery.