Radar-derived interferometric surface currents and their relationship to subsurface current structure

Radar‐derived ocean surface currents are analyzed in conjunction with in situ acoustic Doppler current profiler (ADCP) measurements. The interferometric measurements were collected by an X‐band imaging Doppler radar in a manner analogous to those of along‐track interferometric synthetic aperture radar (ATI‐SAR). While the advent of ATI‐SAR has provided a new, potentially powerful technique for current mapping, the relationship between surface currents and interferometric velocity measurements 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 influence of Bragg resonant waves are studied. We find that coupling between the orbital velocity and backscattered power (i.e., the modulation transfer function) can bias surface current estimates, potentially by up to 20 cm s−1 in an upwind viewing orientation. Furthermore, experimental observations verify a cos2n (θ/2) analytical model for the directional spreading of Bragg resonant waves. Extending our analysis to include subsurface currents, case studies are presented under varying environmental conditions for which the vertical current structure changes considerably. Analysis of radar imagery yields both radial surface currents and vector subsurface current estimates derived from long wave dispersion characteristics. Combining these with coincident 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 interferometric measurements to wind‐drift and the near‐surface current structure.