S. N. Kulichkov

Simulating the influence of an atmospheric fine inhomogeneous structure on long-range propagation of pulsed acoustic signals

The results of simulating the influence of an atmospheric fine structure on the characteristics of acoustic signals propagating throughout the atmosphere for long distances from their sources are presented. A numerical model of an atmospheric fine inhomogeneous structure within the height range z = 20…120 km is proposed to perform calculations. This model and its numerical parameters are based on the current notions of the formation of an atmospheric fine structure due to internal gravity waves. The numerical calculations were performed using the parabolic-equation method. A spatial structure of the acoustic field and the structure of an acoustic signal at long distances from a pulsed source were calculated. It is shown that the presence of an atmospheric fine structure results in a scattering of acoustic signals and their recording in the geometric shadow region. The results of calculations of signal forms are in a satisfactory agreement with data on signals recorded in the geometric shadow region which is formed at a distance of about 300 km from an experimental explosion.

Infrasound Scattering from Atmospheric Anisotropic Inhomogeneities

A model of anisotropic fluctuations forming in wind velocity and air temperature in a stably stratified atmosphere is described. The formation mechanism of these fluctuations is associated with the cascade transport of energy from sources of atmospheric gravity waves to wave disturbances with shorter vertical scales (than the scales of the initial disturbances generated by the sources) and, at the same time, with longer horizontal scales. This model is used to take into account the effects of infrasonic-wave scattering from anisotropic inhomogeneities of the effective sound speed in the atmosphere. Experimental data on the stratospheric, mesospheric, and thermospheric arrivals of signals (generated by explosion sources such as surface explosions and volcanoes) in the zones of acoustic shadow are interpreted on the basis of the results of calculations of the scattered infrasonic field in the context of the parabolic equation. The signals calculated with consideration for the fine structure of wind velocity and air temperature are compared with the signals observed in a shadow zone. The possibility to acoustically sound this structure at heights of both the middle and upper atmospheres is discussed.

On acoustical impulse propagation in a moving inhomogeneous atmospheric layer

In the present work a solution is obtained by the method of normal modes, which describes the sound field of a point impulse source in the atmosphere with definite temperature and wind velocity profiles and ground impedance. This solution describes the impulse shape transformation in different directions relevant to wind velocity involving the low‐frequency case, when the ray approach becomes invalid. The impulse shape evolution during guided and antiguided propagation (in the shadow region) is in good qualitative agreement with the shape observed in the experiment. The change of the impulse shape in time is observed in the experiment during transition of the boundary atmospheric layer from a convective state to one with a stable temperature inversion. The experimental measurements and theoretical estimations show that the nearground acoustical waveguide is formed mainly due to wind stratification in the boundary layer. Solving of the direct problem also shows a principal possibility of inclined sounding ...

Acoustic pulse propagation through a fluctuating stably stratified atmospheric boundary layer

Mesoscale wind speed and temperature fluctuations with periods from 1 min to a few hours significantly affect temporal variability and turbulent regime of the stable atmospheric boundary layer (ABL). Their statistical characteristics are still poorly understood, although the knowledge of such statistics is required when modeling sound propagation through the stable ABL. Several field experiments have been conducted to study the influence of mesoscale wind speed fluctuations on acoustic pulse propagation through the stable ABL. Some results of these experiments are described in this paper. A special acoustic source was used to generate acoustic pulses by the detonation of an air-propane mixture with a repetition period 30 s. The mean wind speed and temperature profiles were continuously measured by Doppler sodar and temperature profiler, whereas mesoscale wind fluctuations were measured by anemometers placed on a 56-m mast. From the measurements of the pulse travel time fluctuations at different distances from the source, the statistical characteristics of the mesoscale wind fluctuations, such as frequency spectra, coherences, horizontal phase speeds and scales, have been obtained. Some of the obtained results are interpreted with the use of a recently developed model for the internal wave spectrum in a stably stratified atmosphere.

On acoustic N -wave reflections from atmospheric layered inhomogeneities

The nonlinear propagation of acoustic pulses from a point source of an explosive character (surface explosion or volcano) throughout the atmosphere with stratified wind-velocity and temperature inhomogeneities is studied. The nonlinear distortions of acoustic pulse and its transformation into an N-wave during its propagation to the upper atmosphere are analyzed in the context of a modified Burgers’ equation which takes into account a geometric ray-tube divergence simultaneously with an increase in both nonlinear and dissipative effects with height due to a decrease in atmospheric density. The problem of reflection of a spherical N-wave from an atmospheric inhomogeneous layer with model vertical wind-velocity and temperature fluctuations having a vertical spectrum that is close to that observed within the middle atmosphere is considered. The relation between the parameters (form, length, frequency spectrum, and intensity) of signals reflected from an atmospheric inhomogeneous layer and the parameters of the atmospheric fine layered structure at reflection heights is analyzed. The theoretically predicted forms of signals reflected from stratified inhomogeneities within the stratosphere and the lower thermosphere are compared to the observed typical forms of both stratospheric and thermospheric arrivals from surface explosions and volcanoes in the zones of an acoustic shadow.

Modeling propagation of infrasound signals observed by a dense seismic network

The long-range propagation of infrasound from a surface explosion with an explosive yield of about 17.6 t TNT that occurred on June 16, 2008 at the Utah Test and Training Range (UTTR) in the western United States is simulated using an atmospheric model that includes fine-scale layered structure of the wind velocity and temperature fields. Synthetic signal parameters (waveforms, amplitudes, and travel times) are calculated using parabolic equation and ray-tracing methods for a number of ranges between 100 and 800 km from the source. The simulation shows the evolution of several branches of stratospheric and thermospheric signals with increasing range from the source. Infrasound signals calculated using a G2S (ground-to-space) atmospheric model perturbed by small-scale layered wind velocity and temperature fluctuations are shown to agree well with recordings made by the dense High Lava Plains seismic network located at an azimuth of 300° from UTTR. The waveforms of calculated infrasound arrivals are compared with those of seismic recordings. This study illustrates the utility of dense seismic networks for mapping an infrasound field with high spatial resolution. The parabolic equation calculations capture both the effect of scattering of infrasound into geometric acoustic shadow zones and significant temporal broadening of the arrivals.

Physical modeling of long-range infrasonic propagation in the atmosphere

The results of experiments on the physical modeling of long-range infrasonic propagation in the atmosphere are given. Such modeling is based on the possible coincidence between the forms of the vertical profiles of the effective sound speed stratification in the atmospheric boundary layer (between 0 and 600 m for the case under consideration) and in the atmosphere as a whole (from the land surface up to thermospheric heights (about 150 km)). The source of acoustic pulses was an oscillator of detonation type. Owing to the detonation of a gas mixture of air (or oxygen) and propane, this generator was capable of producing short, powerful (the maximum acoustic pressure was on the order of 30 to 60 Pa at a distance of 50 to 100 m from the oscillator), and sufficiently stable acoustic pulses with a spectral maximum at frequencies of 40 to 60 Hz and a pulsing period of 20 to 30 s. The sites of acoustic-signal recording were located at different distances (up to 6.5 km) from the source and in different azimuthal directions. The temperature and wind stratifications were monitored in real time during the experiments with an acoustic locator—a sodar—and a temperature profiler. The data on the physical modeling of long-range sound propagation in the atmosphere are analyzed to verify the physical and mathematical models of predicting acoustic fields in the inhomogeneous moving atmosphere on the basis of the parabolic equation and the method of normal waves. A satisfactory agreement between calculated and experimental data is obtained. One more task was to compare the theoretical relations between variations in the azimuths and angles of tilting of sound rays about the horizon and the parameters of anisotropic turbulence in the lower troposphere and stratosphere with the experimental data. A theoretical interpretation of the experimental results is proposed on the basis of the theory of anisotropic turbulence in the atmosphere. The theoretical and experimental results are compared, and a satisfactory agreement between these results is noted.

Characteristics of a fine vertical wind-field structure in the stratosphere and lower thermosphere according to infrasonic signals in the zone of acoustic shadow

Results obtained from the acoustic sounding of a fine layered wind-field structure in the stratosphere, mesosphere, and lower thermosphere with the use of infrasonic waves from surface explosions and volcanic eruptions are given. These results were obtained using a new method of acoustic sounding of the atmosphere based on the phenomenon of infrasound scattering from anisotropic wind-velocity and temperature inhomogeneities into the zone of acoustic shadow. This method makes it possible to obtain data on the vertical wind-velocity structure and its time variability at lower thermospheric heights that are less accessible to other remote methods of atmospheric sounding, including both meteor (up to a height of 105 km) and satellite measurements.

On Experience in using the remote acoustic method of partial reflections in studies of the lower troposphere

Using the phenomenon of the partial reflection of acoustic waves from anisotropic wind-velocity and temperature inhomogeneities in the lower troposphere is justified in determining the structure of these inhomogeneities. The data (obtained with the method of bistatic acoustic sounding) on signals reflected from stratified inhomogeneities in the lower 600-m layer of the troposphere are given. A detonation-type pulsed acoustic source was used. The methods of isolating a small (in amplitude) reflected signal against the background of noise and determining the reflecting-layer height and the partial-reflection coefficient from the measured parameters (time delay and amplitude) of a reflected signal are presented. The method of estimating the vertical gradients of the effective sound speed and the squared acoustic refractive index from the partial-reflection coefficient previously calculated is described on the basis of an Epstein transition-layer model. The indicated parameters are experimentally estimated for concrete cases of recording reflected signals. A comparison of our estimates with independent analogous data simultaneously obtained for the same parameters with monitoring instruments (a sodar and a temperature profiler) has yielded satisfactory results.

Effects that internal gravity waves from convective clouds have on atmospheric pressure and spatial temperature-disturbance distribution

Experimental data on variations in atmospheric surface pressure in the region of thunderstorm phenomena are analyzed. A relationship between variations in atmospheric pressure at the land surface and those in tropospheric temperature has been found, and the relation between the vertical distribution of tropospheric temperature and variations in atmospheric pressure at the land surface is studied. The propagation of internal gravity waves caused by atmospheric heating due to water-vapor condensation during the formation of a convective cloud is simulated. The results of calculations show that the lifetime of these internal gravity waves may significantly exceed the lifetime of this cloud. It is shown that the form of the disturbance of atmospheric pressure under such a convective cloud is a sequence of minimum and maximum pressure variations and the amplitude of maxima may exceed that of minima.