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Acoustic sounding of the lower troposphere
Journal of Atmospheric and Terrestrial Physics, 1968, Voh 30, pp. 1439-1440. Pergamon Press. Printed In Northern Ireland
SHORT P A P E R
Acoustic sounding of the lower troposphere L. G. MCALLISTER Australian Defence Scientific Service, Department of Supply, Weapons Research Establishment, Adelaide, South Australia (Received 3 April 1968) Abstract--An acoustic sounding experiment is described in which pulses of acoustic energy
were directed upwards and reflected from inhomogeneities in the temperature structure of the lower troposphere to obtain a continuous record of their height, movement, and vertical distribution. INTRODUCTIOlq THE VELOCITY of propagation of sound waves through the atmosphere is, to a first approximation, a function of the absolute temperature of the air. Therefore, given an acoustic sounding system (GILL~AN, COXHEAD and WILLIS, 1946) of sufficient sensitivity and an integrating recorder, it is possible to obtain echoes from temperature discontinuities in the lower atmosphere, and so to determine the structural characteristics of one of the key atmospheric parameters as a continuous function of height and time. EXPERIMENT An acoustic sounding experiment was set up at the Weapons Research Establishment, on a coastal plain eighteen miles north of Adelaide in South Australia. The site was subjected to periodic changes from continental to maritime air with the passage of cold fronts which moved in from the Great Australian Bight. Short pulses (50 ms) of acoustic energy at a carrier frequency of 900 Hz, and repeated every 10 secs, were beamed upward from an array of 196, 8 in., commercial loudspeakers. The peak pulse power was 500 W. The array, which was also used as the receiving transducer, was made in four sections (Fig. 1) for ease of transport and was protected from the weather b y sheets of thin plastic stretched across the loudspeakers. Wooden side plates were fitted to each unit to reduce the horizontal spread of the sound. The array was switched from the t r a n s m i t to the receive mode of operation b y electromagnetic relays. The reflected pulses were amplified in an audio amplifier with a bandwidth of 100 Hz and recorded on a facsimile receiver in which the rotation of the helix drum was synchronised with the t r a n s m i t t e d pulse to give an effective height range of 5500 ft. The recording paper (8 in. wide) moved through the recorder at the rate of 3 in./hr, which allowed a full day's record to be compressed into a 6 ft length of record. 1439
1440
L . G . MC~T.T.+STF~R
PRELIMINARY RESULTS The record shown in Fig. 2 was obtained when a ridge of high pressure covered the Great Australian Bight and was pushing in over South Australia. A strong temperature inversion (6°C) had formed at 3000 ft and the lapse rate of temperature below the inversion was close to the dry adiabatic lapse rate. The vertical resolution of the instrument can be gauged from the vertical thickness of the return from the permanent echo at 530 ft. The vertical spread of the returns from the inversion at 3000 ft therefore suggests that these were obtained from a turbulent region inside the inversion layer. While these were the dominant returns, energy was reflected throughout the region characterized b y a d r y adiabatic lapse rate i.e. up to 3000 ft. In contrast, few returns were obtained from the region above 3000 ft. Figure 3 shows the structure of a shallow b u t stable layer of maritime air, undercutting a moist continental air stream. Ground wind was light during this time. The sharply defined boundaries between the contrasting layers are evident (05.15 CST, 1500 ft) as are the vertical oscillations of the b o u n d a r y surface. The strong coupling which existed between the vertical motions of the individual b o u n d a r y surfaces, even though mixing across the b o u n d a r y was suppressed, can also be seen. Returns were obtained from inhomogeneities through the height range, 120 to 2500 ft. Figure 4 was obtained when a high pressure cell was centred south of Adelaide and the winds were south-easterly, with the temperature rising due to modification of the maritime airstream. While the ground conditions were calm, the sheet of turbulent air between 1000 and 2000 ft is clearly seen. CONCLUSIONS The feasibility of obtaining a continuous record of the height, movement and spatial distribution of inhomogenities in the temperature structure of the lower troposphere b y means of acoustic sounding has been demonstrated. With the passage of time further developments of the technique and their applications in other areas will almost certainly occur. PARAMETERS OF ACOUSTIC SOUNDER Carrier frequency 900 Hz Pulse length 50 ms Pulse repetition rate 0.1 pulses/sec Transmitted power 500 W Transmit/receiver transducer Square array of 196, 8 in. loudspeakers Receiver Tuner audio amplifier, 100 Hz bandwidth Recorder Facsimile Receiver 1. Paper speed 3 in./hr 2. Paper width 8in. G ~ . a ~ w G. W., COXaIEAD H. B., Wrr.T.IS F. H.
REFERENCES 1946 J. Acoust. Soc. Am . 18, 274.
Fig. 1. Photograph of tho transmitting and receiving acoustic array.
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SHORT P A P E R
Acoustic sounding of the lower troposphere L. G. MCALLISTER Australian Defence Scientific Service, Department of Supply, Weapons Research Establishment, Adelaide, South Australia (Received 3 April 1968) Abstract--An acoustic sounding experiment is described in which pulses of acoustic energy
were directed upwards and reflected from inhomogeneities in the temperature structure of the lower troposphere to obtain a continuous record of their height, movement, and vertical distribution. INTRODUCTIOlq THE VELOCITY of propagation of sound waves through the atmosphere is, to a first approximation, a function of the absolute temperature of the air. Therefore, given an acoustic sounding system (GILL~AN, COXHEAD and WILLIS, 1946) of sufficient sensitivity and an integrating recorder, it is possible to obtain echoes from temperature discontinuities in the lower atmosphere, and so to determine the structural characteristics of one of the key atmospheric parameters as a continuous function of height and time. EXPERIMENT An acoustic sounding experiment was set up at the Weapons Research Establishment, on a coastal plain eighteen miles north of Adelaide in South Australia. The site was subjected to periodic changes from continental to maritime air with the passage of cold fronts which moved in from the Great Australian Bight. Short pulses (50 ms) of acoustic energy at a carrier frequency of 900 Hz, and repeated every 10 secs, were beamed upward from an array of 196, 8 in., commercial loudspeakers. The peak pulse power was 500 W. The array, which was also used as the receiving transducer, was made in four sections (Fig. 1) for ease of transport and was protected from the weather b y sheets of thin plastic stretched across the loudspeakers. Wooden side plates were fitted to each unit to reduce the horizontal spread of the sound. The array was switched from the t r a n s m i t to the receive mode of operation b y electromagnetic relays. The reflected pulses were amplified in an audio amplifier with a bandwidth of 100 Hz and recorded on a facsimile receiver in which the rotation of the helix drum was synchronised with the t r a n s m i t t e d pulse to give an effective height range of 5500 ft. The recording paper (8 in. wide) moved through the recorder at the rate of 3 in./hr, which allowed a full day's record to be compressed into a 6 ft length of record. 1439
1440
L . G . MC~T.T.+STF~R
PRELIMINARY RESULTS The record shown in Fig. 2 was obtained when a ridge of high pressure covered the Great Australian Bight and was pushing in over South Australia. A strong temperature inversion (6°C) had formed at 3000 ft and the lapse rate of temperature below the inversion was close to the dry adiabatic lapse rate. The vertical resolution of the instrument can be gauged from the vertical thickness of the return from the permanent echo at 530 ft. The vertical spread of the returns from the inversion at 3000 ft therefore suggests that these were obtained from a turbulent region inside the inversion layer. While these were the dominant returns, energy was reflected throughout the region characterized b y a d r y adiabatic lapse rate i.e. up to 3000 ft. In contrast, few returns were obtained from the region above 3000 ft. Figure 3 shows the structure of a shallow b u t stable layer of maritime air, undercutting a moist continental air stream. Ground wind was light during this time. The sharply defined boundaries between the contrasting layers are evident (05.15 CST, 1500 ft) as are the vertical oscillations of the b o u n d a r y surface. The strong coupling which existed between the vertical motions of the individual b o u n d a r y surfaces, even though mixing across the b o u n d a r y was suppressed, can also be seen. Returns were obtained from inhomogeneities through the height range, 120 to 2500 ft. Figure 4 was obtained when a high pressure cell was centred south of Adelaide and the winds were south-easterly, with the temperature rising due to modification of the maritime airstream. While the ground conditions were calm, the sheet of turbulent air between 1000 and 2000 ft is clearly seen. CONCLUSIONS The feasibility of obtaining a continuous record of the height, movement and spatial distribution of inhomogenities in the temperature structure of the lower troposphere b y means of acoustic sounding has been demonstrated. With the passage of time further developments of the technique and their applications in other areas will almost certainly occur. PARAMETERS OF ACOUSTIC SOUNDER Carrier frequency 900 Hz Pulse length 50 ms Pulse repetition rate 0.1 pulses/sec Transmitted power 500 W Transmit/receiver transducer Square array of 196, 8 in. loudspeakers Receiver Tuner audio amplifier, 100 Hz bandwidth Recorder Facsimile Receiver 1. Paper speed 3 in./hr 2. Paper width 8in. G ~ . a ~ w G. W., COXaIEAD H. B., Wrr.T.IS F. H.
REFERENCES 1946 J. Acoust. Soc. Am . 18, 274.
Fig. 1. Photograph of tho transmitting and receiving acoustic array.
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