Elizabeth Twarog

Remote Sensing of Coastal Regions Using an Airborne Imaging Radar

Monday, December 7, 1998
11:00 AM
106 Egan

Abstract

This thesis presents dual-polarized radar scattering measurements of coastal oceanic fronts collected with an experimental X-band airborne imaging radar. The measurements were collected with the radar system installed on a P-3 aircraft over-flying the Chesapeake Bay Outflow Plume. This plume is formed by a layer of buoyant fresh water that tidally cycles in and out of the Chesapeake Bay estuary and lies on top of the denser, more saline water of the continental shelf. The boundary between the plume and the shelf waters is an ocean front characterized by a strong spatial salinity gradient and a convergence between surface currents. These effects roughen the ocean surface at the front, making the frontal boundary detectable against the background waves in radar imagery. Radar backscatter from the fronts are shown to exhibit similar cross sections for horizontal and vertical polarizations indicating that the scatterers responsible for the frontal echo are associated with a non-Bragg mechanism, similar to the scattering from breaking waves.

Scattering from the ambient, or background, waves distant from the fronts, and in the absence of strong winds and currents, is well described by composite- surface scattering theory. The contrast between the frontal echo and the polarization-sensitive ambient wave echo is significantly larger at horizontal radar polarization than at vertical polarization, indicating that horizontally polarized radars are better suited to imaging ocean fronts than vertically polarized radars.

Analysis of temporal and spatial variations in background wave echo collected at a 10 degree grazing angle reveals that these echoes are strongly related to ocean surface current velocity near the peak ebb phase of the tidal cycle. Large tidal currents are found to produce anisotropic surface roughness elements having radar echoes that cannot be explained by conventional composite surface scattering models, but, like the frontal echoes, are indicative of a non-Bragg scattering mechanism, such as small "bore" features located near the crests of breaking waves.

Thesis Committee:
Prof. David J. McLaughlin (advisor)
Prof. D.H. Brooks
Prof. Ph. Serafim
Dr. M. Sletten, Naval Research Laboratory