Evan K. Westwood
Appl. Res. Lab., P.O. Box 8029, The Univ. of Texas at Austin, Austin, TX 78713-8029
An approach has been developed for determining the modal wavefronts that allow normal modes to be propagated adiabatically in smoothly range-dependent ocean waveguides. The method is motivated by the fact that propagation in an isospeed penetrable wedge is accurately modeled by ``adiabatic wedge modes,'' which are identical to the usual ``vertical modes'' except that the wavefront curvature induced by the sloping bottom is taken into account. For the wedge, the wavefronts are assumed to be circular arcs centered on the wedge apex. In order to generalize the notion of wedge modes to other types of range dependence, wavefronts are constructed numerically such that the derivative of each mode in the direction normal to its wavefront, (cursive beta)(phi)/(cursive beta)s, is zero. The result is that the first-order modal coupling coefficients, which involve (cursive beta)(phi)/(cursive beta)s integrated over depth, are zero, and the adiabatic algorithm can be used. The quantities (cursive beta)(phi)/(cursive beta)z and (cursive beta)(phi)/(cursive beta)r versus depth are used to obtain the directions normal to the wavefront. For the isospeed penetrable wedge, wavefronts computed in this manner do indeed correspond closely to the expected circular arcs. [Work supported by the ARL Internal Research and Development Program.]