Coupled volume and surface scattering

The oceanography of the near‐surface zone of the temperate oceans is characterized by a mixed layer of varying depth and a sharp thermocline with large sound velocity gradients. Most studies of surface scattering have ignored the effect of the coupling between volume scattering and scattering from the rough surface. Recent Navy emphasis on ocean surface scattering measurements has also ignored the volume effects. The gradients at the bottom of a mixed layer can be large enough to cause wide variations in the transmission and reflection coefficients (including total internal reflection) for the shallow angles of long‐range propagation. The wave packets that are found on the sharp thermocline can have large displacements and space‐time scales near those of surface waves, but with differing dispersion relations. A wide‐angle PE code has been used to scatter sound from an ocean surface with a Pierson‐Moskowitz spectrum, and the effects of the randomness on the scattered field with and without thermocline waves have been studied. The PE code used was developed in a joint project with Eric Thorsos (APL‐UW). The internal waves were generated to be similar in scale to observations with a towed chain (Marmorino et al., J. Geophys. Res. 92 C12, 13049–13062). The results indicate that volume scattering cannot be ignored when modeling shallow‐angle surface scattering.