The effects of turbulent intermittency on scattering and estimates for the degree of saturation

Abstract

Turbulent fluctuations in atmospheric wind and temperature fields are observed to be erratic in time; strong activity is typically interspersed with periods of relative calm. This property, referred to as turbulent intermittency, has an observable effect on the statistics of scattered acoustic signals. For fully saturated scattering, large intensity deviations about the mean result in a divergence from log-normal intensity probability density functions (pdfs) typically used to describe such statistics. Previous studies have developed the theory to predict the intensity pdf’s that account for both turbulent intermittency and the degree of saturation [Norris et al., J. Acoust. Soc. Am. 109, 1871–1880 (2001); Wilson et al., J. Acoust. Soc. Am. 99, 3393–3400 (1996)]. The new formulation with intermittency is compared to the generalized gamma pdf previously proposed for propagation in random media. It is also compared to data collected over a 140 m line-of-sight path at 110 to 525 Hz. Experimental characterization of the pdf parameters relating to intermittency and signal saturation is attempted with partial success. Strength/diffraction parameters and complex signal moments are computed to further define the scattering effects as a function of frequency and help identify limitations in the theory.