Spectrum of measured infrasonic emissions from clear air turbulence

An array of three infrasonic microphones (0.2–20 Hz), operating continuously in the field at NASA Langley Research Center, on several occasions received a class of signals interpreted as infrasonic emissions from clear air turbulence. The presence and location of the turbulence were confirmed by pilot reports (PIREPS), and the direction of emitted signals toward the array was determined by slowness mapping. The coherence of the signals among the three microphone pairs in the array was close to unity. The amplitude spectrum of the received signals was found to fit a power law having an exponent of −7/2, which disagrees with the exponent of −7/4 of Meecham and Ford [J. Acoust. Soc. Am. 30, 318–322 (1958)], based on turbulence self-noise and with the exponent of −1 of Meecham [J. Acoust. Soc. Am. 33, 149–155 (1971)], based on mean shear fluctuations. Thus the above models do not account for the observed spectrum. Two case histories are described in detail.