Abstract
Two- and four-layer models of the atmosphere up to heights of about 500 km allow a systematic and fairly accurate account of the far-field properties of guided internal and surface gravity waves having periods >10 min. Simple formulae and numerical results are given for a variety of models allowing one to determine the importance of such effects as compressibility, free vs rigid boundaries, layering, and the earth's rotation. The importance of coupling effects similar to those occurring in layered acoustic and electromagnetic waveguides is emphasized. It is also shown, in the period pass-band between 10 and 200 min, that there is in all models a difference between surface and internal wave group velocities of sufficient magnitude to preclude confusion in the travel times of these modes. It is also stressed that the layer of atmosphere between altitudes of 110 and 150 km, in which the Väisälä frequency may exceed the acoustic cut-off frequency, plays a critical role in determining the amplitude of the ground-level pressure perturbations associated with the passage of a surface gravity wave. This amplitude is sensitive to the precise laws of atmospheric stratification as well as to wind sheer fields at these heights. It is shown that our assumptions concerning the nature of the “effective free surface” of the atmosphere are at least consistent with the damping of 15 min period, 600 m sec−1 pressure waves observed in connection with large nuclear explosions in 1967 and 1968.
