Infrasonic Fluctuation Spectra in the Atmosphere

Summary The concept of normal co‐ordinates furnishes a convenient and powerful method for calculating fluctuation spectra in linear media. The procedure is illustrated by two examples from atmospheric infrasonics, in which it is assumed that random forces set up fields of subsonic gravity waves in stratified fluids. The first example, a model of convective generation of waves by upward moving thermals, predicts power spectra of the type and magnitude that have been observed, in the 5 min to 1 hr bandpass, at times of strong convection. The second example discusses the generation of waves in the same bandpass by a horizontally moving field of random vertical forces, and may be viewed as a rough model of the effect of a turbulent layer in a tropospheric wind system; after introduction of a critical layer effect the calculations can be made to agree with measurements of jetstream generated pressures at ground level. In both examples it is confirmed that the local value of the Väisälä frequency N plays a decisive role in shaping the spectrum; in the convective case it determines a peak frequency ω 0 which, however, can be strongly shifted towards lower values ω 0 < N by the influence of the input spectrum (i.e. the properties of the random force field); in the turbulent layer model, it actually determines rather well the frequency of a quite generally observed break in the spectrum curve although, here again, this break occurs at frequencies somewhat less than N .