R.S. Cole

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 CT2 can be derived from the returns of a monostatic acoustic sounder in both stable and convective atmospheric conditions.

High frequency atmospheric acoustic sounders

Abstract The use of a relatively high frequency for atmospheric sounding is discussed. Details are given of two acoustic sounders operating at frequencies of 4.7kHz and 6.5kHz. Conventional acoustic sounders usually operate at frequencies below about 2kHz. This is primarily because the atmospheric attenuation of sound at these frequencies is relatively small and greater acoustic output powers can be obtained from a given size of antenna in this frequency range. However, there are advantages to be gained from the use of sounders operating at somewhat higher frequencies.For example, the performance of a sounder can often be degraded by environmental noise, such as that produced by traffic and aircraft. This was found to be a limitation in the case of a sounder operating at 1730Hz (Asimakopoulos and Cole 1977) at University College London. A high frequency sounder operating at 4.7kHz is found to be less adversely affected by these noise sources because the acoustic spectra of both traffic and aircraft noise contain less energy at higher frequencies. The reverberation time of the acoustic antenna after the transmission of a pulse determines the minimum height from which an echo can be received. This time is shorter for a physically smaller high frequency antenna and hence a region of the atmosphere much closer to the ground can be probed.

Design of arrays for acoustic sounder antennas

The use of an array of loudspeaker elements for an acoustic sounder antenna is discussed. The effect of variations between the phase and amplitude of individual elements in the array is investigated and measurements are compared with the predicted radiation patterns for both a 6*6 square array and a 2*12 rectangular array of re-entrant horn loudspeakers.

A comparison of acoustic Doppler vertical velocity power spectra with direct measurements

Abstract Power spectra of vertical wind velocity obtained using a monostatic acoustic sounder are compared with directly measured spectra at heights of 56 m, 86 m and 124 m in the planetary boundary layer. Encouraging agreement is found between the two methods of measurement, under conditions of low horizontal wind speed. Variances of the vertical velocity estimates are within 30% across the height range of these measurements.

Temperature structure parameter measurements using differential temperature sensors

The comparison of CinfTsup2 estimates in the atmospheric boundary layer, from spectral and differential temperature (δT) measurements, is discussed. Measurements of CinfTsup2 using these two methods are compared and the differences between the two are shown to be due to low-frequency enhancement of the δT spectrum. Possible explanations for this effect are considered and attention is drawn to the significance of the resulting errors in boundary-layer turbulence measurements.

A general radar equation for the bistatic acoustic sounder

A radar equation which can be applied to any bistatic acoustic sounder is derived. The equation reduces to the expression normally used for the special case of the monostatic sounder. Numerical results using this equation are given for specific acoustic sounders, including the relative contributions to the scattering from the temperature and velocity parameters (CT2 and CV2), the effect of wind speed on the scattered intensity and the measurement of horizontal wind velocity.

The evaluation of acoustic sounder returns from a methane plume

The acoustic scattering cross section of a turbulent methane-air mixture is derived and expressed in terms of a concentration structure parameter Cm2. This parameter can be estimated from the backscattered echo of an atmospheric acoustic sounder and hence variations in the concentration of the methane in the plume determined. Simple diffusion theory, involving atmospheric parameters which can be determined by use of an acoustic sounder, is used to predict the size of the plume. The theoretical predictions are compared with experimental results.

Simultaneous horizontal and vertical acoustic sounding of the atmospheric boundary layer

Some preliminary results from a series of experiments of simultaneous horizontal and vertical acoustic sounding of the atmosphere are described and discussed. The novel horizontal configuration offers some potential for investigating thermal plume motion and characteristics within the boundary layer. It could also be of use in assessing the importance of excess attenuation in quantitative acoustic sounding.

Measurement of humidity using multifrequency atmospheric acoustic sounding

Abstract The measurement of atmospheric humidity profiles using three atmospheric acoustic sounders operating at frequencies of 2048, 4681 and 6554 Hz is described. The problems associated with the measurement of humidity using this method and the errors likely to occur are discussed. The results indicate that a multifrequency sounding system could be used to measure water vapour pressure over a height range of 50–150 m with an error of 1 or 2mb.

The use of the atmospheric acoustic sounder to track methane gas plumes

Abstract The usefulness of the acoustic sounder as a method for tracking methane gas plumes in the atmosphere is investigated. An “equivalent” temperature structure parameter, CT2, of 0.5°C2 m −2 3 for a methane gas plume is obtained. The possibility of using the acoustic sounder to measure relative concentrations of gas in the plume is discussed.