We give a detailed review of developments in atmospheric acoustics of the last decade. These developments include new ways to use refractive effects, studies of phase and amplitude fluctuations during propagation of sound along a path, nonlinear effects near high-powered acoustic antennas, problems related to noise, insights into large-scale atmospheric processes gained from infrasound, applications dependent on the Doppler frequency shift, and hybrid devices using both acoustic and electromagnetic waves. The introduction of the echosonde approximately 10 years ago results in the considerable emphasis given to advances in active acoustic remote sensing of atmospheric structure.

Advances in atmospheric acoustics

In the bistatic configuration, acoustic echoes from the atmospheric boundary layer are determined by the magnitude of the structure parameters for wind and temperature, C and C respectively. Normally the contribution from C is relatively small and so this arrangement offers a remote method for determining C, and hence the rate of dissipation of turbulent energy, ϵ. A quantitative assessment of this technique is presented. Direct estimates of ϵ and C have been made using turbulence probes attached to the tethering cable of a large (1300m3) balloon. Sounder-derived values were obtained using two carefully calibrated acoustic arrays. Results obtained in both convective and stable conditions are described and contrasted.

Quantitative bistatic acoustic sounding of the atmospheric boundary layer

Results are presented which show that reasonably accurate estimates of the atmospheric temperature structure paparmeter C
T
2 can be derived from the returns of a monostatic acoustic sounder in both stable and convective atmospheric conditions.

A quantitative comparison between acoustic sounder returns and the direct measurement of atmospheric temperature fluctuations

Results are presented which show that reasonably accurate estimates of the atmospheric temperature structure paparmeter C$ can be derived from the returns of a monostatic acoustic sounder in both stable and convective atmospheric conditions.

http://rd.springer.com/content/pdf/10.1007%2FBF00229281.pdf

Sounder returns and the direct measurement of atmospheric temperature fluctuations

In this paper, measurements of the first 150 m of the atmospheric boundary layer obtained by a high-frequency acoustic mini-sounder are compared with measurements obtained by a full complement of instruments including sonic anemometers mounted on the Boulder Atmospheric Observatory tower. The acoustic mini-sounder, starting as low as 6 m from the ground, measures in the monostatic mode the profiles of the vertical wind speed, w, and of the temperature structure parameter, CT2 with enhanced height resolution of the order of 1 m and time resolution of the order of 30 s. The results of the comparison show that the high-frequency mini-sounder is an effective atmospheric boundary-layer profiler that is also portable and relatively inexpensive.

Quantitative low-level acoustic sounding and comparison with direct measurements

The successful utilization of acoustic waves for atmospheric echo sounding requires an efficient antenna, with high on‐axis gain and suppressed sidelobes. Existing microwave antennas and optical searchlights have been adapted for acoustic use, and their performance measured. A conical horn reflector antenna made of fiberglass has given excellent performance at 1–5 kHz after being coated with a viscous damping tar to keep reverberation time short for the reception of weak echoes with minimum delay following the transmission of 20 acoustic watts from the antenna. Septum dampers have been used successfully on parabolic dish antennas. Antenna beam patterns have been measured on an outdoor range, across a canyon, and the results agree closely with predictions from diffraction theory. Absorbing, cylindrical antenna enclosures have been found effective in reducing sidelobe reception by 10 to 20 dB, also in agreement with predictions. Attention to antenna design should provide unimpaired echo sounder operation in...

J. W. Wescott

Papers

Acoustic antennas for atmospheric echo sounding