RADAR-DERIVED INTERFEROMETRIC SURFACE CURRENTS AND THEIR RELATIONSHIPTO SUBSURFACE CURRENT STRUCTURE

Radar-derived ocean surface currents are analyzed in conjunction with in situ acoustic Doppler current profiler (ADCP) measurements. The int erferometric measurements were collected by an X-band imaging Doppler radar in a manner analogous to those of along-track interferometric sy nthetic aperture radar (ATI-SAR). While the advent of ATI-SAR has prov ided a new, potentially powerful technique for current mapping, the re lationship between surface currents and interferometric velocity measu rements is not yet clearly understood. This paper presents comparisons between radar-derived and in situ current measurements. To develop a precise method for estimating the surface current from interferometric measurements, the influence of long wave orbital velocities and the i nfluence of Bragg resonant waves are studied. We find that coupling be tween the orbital velocity and backscattered power (i.e., the modulati on transfer function) can bias surface current estimates, potentially by up to 20 cm s(-1) in an upwind viewing orientation. Furthermore, ex perimental observations verify a cos(2n) (theta/2) analytical model fo r the directional spreading of Bragg resonant waves. Extending our ana lysis to include subsurface currents, case studies are presented under varying environmental conditions for which the vertical current struc ture changes considerably. Analysis of radar imagery yields both radia l surface currents and vector subsurface current estimates derived fro m long wave dispersion characteristics. Combining these with coinciden t ADCP measurements yields a vertical profile of current. Using these measurement techniques, we make several observations within the upper meter of the ocean. These profiles reveal the sensitivity of X-band in terferometric measurements to wind-drift and the near-surface current structure.