Anomalous vertical sound propagation and the ground-blocking of turbulence

Scattering by turbulence leads to random scintillations and coherence loss of sound waves, among other impacts. In a recent experiment involving transmissions from a 130-m tower to microphones on the ground [Kamrath et al., J. Acoust. Soc. Am. 149(3), 2055–2071 (2021)], the turbulence impacts on phase variances were shown to be considerably smaller for vertical and slanted paths than theoretically predicted. The primary cause is likely ground blocking of the turbulence: since the sound waves are impacted most strongly by the turbulent velocity fluctuations in the direction of propagation, and the ground blocks large vertical motions, the turbulence impacts are relatively weakened for vertical paths. An approach is described for including the ground blocking in the turbulence spectrum and, thus, in acoustic scattering calculations. The anisotropic propagation effect could possibly be used to measure the relative strengths of vertical and horizonal velocity fluctuations in the near-ground atmosphere.