Estimating ocean frontal surface velocity distributions from radar imagery signatures

Abstract

An inversion algorithm for inferring the surface velocity field of buoyant plume frontal features from observed radar imagery has been developed. The inversion technique is based upon an assumption, suggested by Alpers and Hennings' (AH) relaxation model (1984), that near strongly convergent fronts, the radar cross-section should be proportional to the component of the local current gradient that is directed along the radar-look direction. However, at X-band, the technique only works when wave-breaking (WB) effects, which are not included in the AH model, are incorporated. This WB model successfully reproduces the magnitude of the signature in images of the plume front at higher frequencies (X-band), where it is known that the AH model is deficient. WB effects play a dominant ro/spl circ/le in the backscatter associated with frontal regions with strong surface convergence fields. These results suggest that the enhancements of radar backscatter in the vicinity of strongly-convergent fronts are proportional to the local current-convergence but that the underlying scattering process involves WB in a manner that cannot be understood from the AH model. Results are presented for the estimated velocity field derived from radar imagery of the Chesapeake Bay plume front. Preliminary considerations of the convergence and uniqueness of the inversion technique are extended by means of a controlled numerical experiment involving the inversion of a prescribed input velocity field.