ACOUSTIC PROPAGATION THROUGH ANISOTROPIC, SURFACE-LAYER TURBULENCE

The effects of atmospheric wind and temperature fluctuations on acoustic signal variability is discussed, with emphasis on the effects of large-scale turbulence (motions having size larger than or comparable to the integral length scale). Such large-scale turbulence is anisotropic, is generated by both shear and buoyancy instabilities, and has structure that depends strongly on the meteorological conditions as well as the distance from the ground. Previous research in the atmospheric sciences literature regarding length scales and anisotropy is reviewed and incorporated into an acoustic propagation model. The model is based on a multiply scaled, six-termed, sound-speed correlation function. A second, simpler model, based on fluctuating curvature of the vertical wind profile, is also proposed. Both models are compared with experimental measurements of amplitude and travel-time fluctuations obtained during the Rock Springs Tomography Experiment, which involved concurrent monitoring of acoustic fluctuations and atmospheric turbulence statistics. An important conclusion is that large-scale turbulence is a significant cause of acoustic signal fluctuations, particularly in the signal phase.