ATMOSPHERIC EDGE WAVES

In an isothermal windless atmosphere Lamb's wave, the energy of which decays exponentially with height, propagates non‐dispersively with the speed of sound. In the real atmosphere the speed of sound and the wind speed vary with height, but it is known that an edge wave similar to Lamb's wave is still possible. Assuming the wind speed and variations in the speed of sound to be much less than some typical sound speed, this atmospheric edge wave is shown to have group velocity given approximately by Where ω is the wave frequency, c̄ the mean of the sound and wind speeds weighted with the energy density of the basic Lamb wave, and D is a positive dispersion coefficient defined in terms of a simple integral of the departures of the sound and wind speeds from their weighted means. Expressions are derived for the dispersive effect of a change in the sound or wind speed at any height, and these are evaluated for a particular model atmosphere. It is shown that the effect of horizontal inhomogeneities in the atmosphere is merely to average the long wave speed and dispersion coefficient along the great circle path from source to receiver. The theory is compared with the results obtained from microbarograms of pressure pulses from large atmospheric explosions, but it is found that the paucity of atmospheric data makes it difficult to use these results to estimate winds above sounding heights on the path from source to receiver. Atmospheric edge waves are shown to be rather insensitive to the upper boundary condition in general, though the effect on microbarograms of coupling between the edge wave and waves propagating high in the atmosphere is discussed, and various decoupling mechanisms, including dissipative decoupling, are described. Copyright © 1969 Royal Meteorological Society