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On the Detectability of Fog, Cloud, Rain and Snow by Acoustic Echo-Sounding Methods

C. Gordon LittleEnvironmental Research Laboratories, NOAA, Boulder, Colo. 80302

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Abstract

The scatter of sound waves by fog, cloud, rain and snow particles is analyzed for the Rayleigh case (i.e., D≪λ). It is shown that relatively standard acoustic echo-sounding equipment should obtain strong echoes (about 49 dB above noise at 300 m range) during heavy snow conditions. Rain at 300 m range would produce echoes varying from about 23 dB to about (58 − Y) dB above noise for conditions ranging from drizzle (1 mm hr−1) to heavy rain (25 mm hr−1), the parameter Y being used to denote the increase in noise level in dB due to precipitation noise. Fogs and clouds with Z⩾5 × 10−2 mm6 m−3 would be detectable out to 300 m range on the standard system.

Acoustic energy scattered from naturally occurring velocity and temperature fluctuations in the boundary layer of the atmosphere will tend to mask the hydrometeor echoes. Three methods of distinguishing hydrometeor echoes from those resulting from irregular velocity and temperature fields are suggested. An optimum experimental configuration to study hydrometeor echoes is proposed.

The fluctuation in acoustic refractive index created by the turbulent wakes of precipitating hydrometeors has also been investigated theoretically; it is concluded that the wakes are too weak to be detected in the presence of the normal spectra of variability of the atmospheric boundary layer.

Abstract

The scatter of sound waves by fog, cloud, rain and snow particles is analyzed for the Rayleigh case (i.e., D≪λ). It is shown that relatively standard acoustic echo-sounding equipment should obtain strong echoes (about 49 dB above noise at 300 m range) during heavy snow conditions. Rain at 300 m range would produce echoes varying from about 23 dB to about (58 − Y) dB above noise for conditions ranging from drizzle (1 mm hr−1) to heavy rain (25 mm hr−1), the parameter Y being used to denote the increase in noise level in dB due to precipitation noise. Fogs and clouds with Z⩾5 × 10−2 mm6 m−3 would be detectable out to 300 m range on the standard system.

Acoustic energy scattered from naturally occurring velocity and temperature fluctuations in the boundary layer of the atmosphere will tend to mask the hydrometeor echoes. Three methods of distinguishing hydrometeor echoes from those resulting from irregular velocity and temperature fields are suggested. An optimum experimental configuration to study hydrometeor echoes is proposed.

The fluctuation in acoustic refractive index created by the turbulent wakes of precipitating hydrometeors has also been investigated theoretically; it is concluded that the wakes are too weak to be detected in the presence of the normal spectra of variability of the atmospheric boundary layer.

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