On the Coherent Reflection of Acoustic Pulses from Anisotropic Structures in the Atmospheric Boundary Layer

The effects of anisotropic irregularities of the acoustic refractive index on propagation and scattering of acoustic pulses in the atmospheric boundary layer (ABL) are investigated. The series of 90 acoustic pulses (called "shots") were generated by special detonational source during different day time periods with a 1 min repetition period. The 3 microphones 50–100 m apart were placed on the ground surface along the straight line with the source and at a distance of about 2.5 km from the source. Certain features were found for the signals received at the distances of about 2.5 km from the source, one of which was a presence of a so-called "head" of the signal, the shape and duration of which was well explained with a ray-mode theory [1] of sound propagation in acoustic wave guides formed by a near-surface mean wind speed and temperature stratification. Another feature was a long "tail" of the signal that followed after the "head" and contained the oscillations of an enhanced intensity , which showed relatively high coherence (∼0.8–0.9) between different microphones [2]. Such highly coherent portions of the tail were assumed to arise from the Fresnel (or partial) reflections of the pulses from highly-anisotropic inhomogeneities of wind speed and temperature in the atmospheric boundary layer. The reflected "tails" of the signals obtained for different "shots" were coherent between each other during 30 min and even longer. The coherent summation of the waveforms of reflected signals for different "shots" have been carried out and showed that the integral signals obtained at different microphones were coherent between each other as well. The results obtained revealed directly a presence of large-scale anisotropic coherent structures in the atmospheric boundary layer.