A numerical study of backscattering from time-evolving sea surfaces: comparison of hydrodynamic models

Results from a Monte Carlo simulation of backscattering from one-dimensional (1-D) time-evolving sea surface models are reported. A numerical electromagnetic method based on an accelerated forward-backward approach is used to calculate backscattered returns from impedance surface profiles at incidence angles of 0/spl deg/ (normal), 40/spl deg/, and 80/spl deg/. Surfaces are initialized as realizations of a Pierson-Moskowitz spectrum and then stepped in time through a numerical hydrodynamic method. Results from three distinct hydrodynamic methods are compared: a linear evolution, the "improved linear representation" of Creamer et al. (1989), and the "Watson-West" approach of West et al. (1987). Instabilities in the West model due to formation of steep wave features limit the study to L-band backscattering for wind speeds less than 2 m/s, so that the surfaces considered are only slightly rough on an electromagnetic scale. The small slope approximation for electromagnetic scattering is shown to provide reasonable predictions in this limit. Statistics of the resulting surface profiles and backscattered fields are compared for the three models and are found to be similar in most respects. Backscattered field Doppler spectra, however, show differences, with the West model apparently capturing more nonlinear interactions in the surface evolution.