RADAR SURFACE SIGNATURES BASED ON THE 2-DIMENSIONAL TIDAL CIRCULATION OF PHELPS BANK

Numerical results for the M2 tidal currents over Phelps Bank, Nantucket Shoals, Massachusetts, with 1/8 x 1/8 min arc resolution are used to predict the radar surface signatures (cross sections-sigma) of Phelps Bank using the Alpers and Hennings [11] first-order relaxation model. The numerical predictions for sigma-incorporate all two-dimensional contributions from the tidal currents and include both the residual and first harmonic term. Not unexpectedly, strong correlation between spatially resolved contour plots of sigma over a wide range of directions and time intervals within the tidal cycle, with variations in the bottom topography of the Phelps Bank, is obtained even when Bragg wave scattering alone is included. Additional contributions to the radar surface signatures should come from other scattering mechanisms (i.e., specular, tilting, wave-breaking, etc.) as well as from hydrodynamic effects such as the shoaling of long ocean waves over the bottom features modulating the energy of the Bragg resonant waves. The predictions for sigma are contrasted with previous measurements in the Phelps Bank by the Naval Research Laboratory (NRL) and a discussion is given of the importance of including the off-diagonal elements from the current stress tensor in the evaluation of sigma.