RADAR SURFACE SIGNATURES FOR THE 2-DIMENSIONAL TIDAL CIRCULATION OVERPHELPS BANK, NANTUCKET SHOALS - A COMPARISON BETWEEN THEORY AND EXPERIMENT

A comparison is made between real aperture radar (RAR) measurements an d simulations (based on modeled tidal currents) of radar cross section over a complicated tidal basin (in the vicinity of the Phelps Bank re gion of the Nantucket shoals) in order to more fully understand the or igin of radar signatures that are observed at the ocean surface as a c onsequence of variations in the topography of the ocean bottom. The Ph elps Bank region was mapped under two extreme wind speed conditions: i n high winds, in excess of 15 m/s, and in low winds, of the order of 2 -3 m/s. For the light-wind case the measured radar cross section over the west side of the Phelps Bank was enhanced by as much as 20 dB rela tive to the clutter background. For the high-wind case, no discernible bathymetric signature was found in the high-clutter background. Numer ical results for the two-dimensional M2 (semidiurnal) tidal currents o ver the Phelps Bank (Greenberg et al., 1989), with 1/8 x 1/8 min of ar c resolution, are used as input to the surface signature models: the A lpers and Hennings (1984) first-order Bragg relaxation model; a genera lized form of this relaxation model (in which wind directional effects are incorporated in an approximate manner); and the full-spectrum mod el of Lyzenga and Bennett (1988). Comparisons between the models (whic h do not include wave breaking) and an extreme case of 2-3 m/s winds ( where strong wave breaking could become important) reveal that althoug h the models predict correlation between variations in bottom topograp hy and surface signature, they significantly underpredict the magnitud e of the observed effect. The model calculations also are very sensiti ve at low (<2 m/s) wind speeds to the functional form that is assumed for the wind-wave forcing in the wave action equation. Prior visual ob servations and measurements of wave spectra (and wave shoaling) in the vicinity of Phelps Bank strongly suggest that the deficiencies of the modeled results that occur explicitly at light winds are due to wave breaking. A number of additional experiments and measurements are sugg ested for more normal environmental conditions for further theory asse ssments.