High-frequency propagation in an elevated tropospheric duct

High-frequency propagation in an elevated tropospheric duct above the earth is analyzed conventionally by normal mode summation, which may be inefficient and often of questionable accuracy because of the large number of modes required. Ray-optical methods, which have been employed, are likewise subject to limitations. The present study explores a new method that combines a judiciously chosen mixture of ray-optical and normal mode fields plus a remainder field that is usually small. The modes may be guided along the concave side of the duct boundary or between this boundary and the earth's surface. By retaining the advantages of both the modal and ray formulations, the hybrid scheme generally requires far fewer rays and modes than the number of modes in a conventional normal mode representation. The hybrid method is applied here to the Green's function problem for a two-dimensional model of the duct formed by a refractive index discontinuity above a perfectly conducting earth. Field values for various locations of source and observation points are computed from the normal mode formulation, which is used as a reference solution, and from the hybrid formulation. The features of these alternative computational methods are discussed in detail and reveal the advantages of the hybrid scheme. Physical interpretation of the hybrid ray-mode field is emphasized throughout and grants new insight into various wave processes.