Excess attenuation in echosonde signals

Abstract

The scattering of sound by turbulence redistributes the acoustic energy flow in space. For sound propagation with a given geometry, such a redistribution can appear as an energy loss in the received part of a beam. Such a loss now is called excess attenuation. The following analysis determines the amount of excess attenuation in the signal obtained in the configuration of a typical monostatic echosonde. Such estimates of excess attenuation are of great importance for accurate quantitative acoustic remote sensing of atmospheric parameters.

Quantitative low-level acoustic sounding and comparison with direct measurements

“…Equation (5) does not take into account either excess attenuation, i.e., the loss of acoustic intensity that results from refraction by the wind (Neff, 1978;Moulsley and Cole, 1980) or turbulent beam broadening (Neff and Haugen, 1978;Clifford and Brown, 1980;Soom and Gu, 1981). Refraction produces a change in the angle of arrival of the backscattered sound given by (Neff, 1978), $(z) = 2 s 44 ds, CZ 0…”

“…Beam broadening by turbulence is less understood at the moment, and comparison of theoretical predictions with experimental results shows only order-ofmagnitude agreement (e.g., Haugen and Kaimal, 1978;Neff, 1978). However, this effect can be estimated with the theory proposed by Clifford and Brown (1980). The temperature structure parameter, C$, can also be obtained from the expression (Kaimal, 1973),…”

“…The calibration factors obtained were later adjusted by 2-3 dB (the order of accuracy of the calibration) for better agreement between the two sounders and direct estimates of C$ The atmospheric absorption term in Equation (3) was calculated with the method proposed by Sutherland (1975) from BAO tower temperature and humidity measurements. The magnitude of broadening was calculated from measurements of C$ and C: and the theory put forward by Clifford and Brown (1980), to be equal to or less than 3 dB. No correction for this effect was applied to the data presented here because the effects of spatial and temporal variations in the structure parameters cannot be easily included in this model and such work falls outside the scope of this paper.…”

Two-dimensional field of thermal turbulence at the edge of an escarpment

“…Excess attenuation can cause significant systematic errors in measurements of Ct. during convective conditions (Haugen and Kaimal, 1978;Neff, 1978;Mousley et al, 1981). That portion of the tower comparison data taken during strong convective conditions indicates possible excess attenuation in the range of 5-20% between 90 and 150 m. However, since both the velocity structure parameter, Cc, and C$contribute to excess attenuation (Clifford and Brown, 1980) and little is known about the nature of C2, at such a mountain site, it is not possible to estimate the magnitude of excess attenuation at the mountain site. No corrections for excess attenuation have therefore been made; however it is clear that the effects discussed in this paper are sufficiently large that excess attenuation is not expected to affect the conclusions.…”

A quantitative comparison of horizontal and vertical acoustic sounding with in-situ measurements

“…The first two are functions of temperature, humidity and to a lesser extent pressure, and can be accurately calculated from measurements of these parameters (Sutherland, 1975) for any given frequency. Excess attenuation is the term used for loss of acoustic intensity due to refraction by the wind (Neff, 1978;Moulsley and Cole, 1980) and turbulent beam broadening (Brown and Clifford, 1976;Clifford and Brown, 1980). Refraction results in a change in the angle of arrival of the backscattered sound which gives rise to loss of received intensity due to the lower-sensitivity of the receiving antenna off-axis.…”