Reformulation of the parabolic approximation for waves in stratified moving media

An asymptotic, large wave number approximation for the equations governing the propagation of acoustic disturbances through a stratified moving medium is developed. The theory is an extension of the geometric acoustics approximation and provides corrections to that approximation in the form of multiplicative functions that satisfy parabolic differential equations of second order. By properly accounting for variations in the acoustic field in directions normal to the rays, both caustic surfaces and the secularity of the geometric theory may be avoided.