A new rough surface parabolic equation program for computing low-frequency acoustic forward scattering from the ocean surface

The parabolic equation acoustic propagation program RAM has been extended to handle a rough air-water interface treated as a series of stair steps. Because much finer vertical spacing may be needed to resolve the interface than to propagate a low-frequency acoustic wave accurately in the rest of the domain, an option to use one vertical spacing near the surface and a coarser one in the rest of the ocean has been implemented. The necessary Galerkin approximations of derivatives on an unequally spaced grid have been worked out using computer algebra. The new program, Rrsfc, is efficient enough to compute the frequency spectrum of the field scattered off a moving surface by treating the surface as a sequence of frozen surfaces. A comparison of one such spectrum with actual ocean data is provided. In order to make a more detailed assessment of the program, both the depth-dependent pressure for individual surfaces and the frequency spectrum for sequences of surfaces are compared with the results of numerically exact integral equation calculations for a few constant sound speed test cases. (C) 1999 Acoustical Society of America. [S0001-4966(99)02501-1].