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.     

Studying characteristics of a fine layered structure of the lower troposphere on the basis of acoustic pulse sounding

Results of acoustic sounding of the lower troposphere with the aid of detonation generators of acoustic pulses are given. This sounding method is based on a partial reflection of acoustic pulses with shock fronts from vertical wind-velocity and temperature gradients continuously varying with height in the troposphere and on the penetration of reflected signals into the region of acoustic shadow. Experiments on tropospheric sounding were carried out on the ground of the Barva Innovation Scientific and Technical Center (Talin, Armenia) in September 2015. In these experiments, an antihail acoustic system was first used as a generator of acoustic pulses. Experimental results have been compared with data obtained earlier in similar experiments carried out in the vicinity of Zvenigorod with the use of a special detonation generator of acoustic pulses. Due to the high resolution (in height) of the sounding method, which reaches 1 m in the stably stratified lower troposphere within a height range of 250–600 m, the vertical profiles of layered effective sound speed inhomogeneities with vertical scales from a few to a few tens of meters have been retrieved. The influence of these fluctuations on the form and amplitude of acoustic signals at a long distance from their pulsed source has been studied.     

The propagation of an acoustic pulse in a near-ground atmospheric waveguide

A wave theory of propagation of an acoustic pulse in a moving stratified atmospheric layer above the ground with a finite impedance of an underlying ground surface is developed. The shapes of acoustic signals in a near-ground atmospheric waveguide, which are formed due to temperature inversion and a vertical shear of the wind velocity, are calculated based on this theory. These signals are compared with those measured during the experiments where vertical profiles of the wind velocity and temperature in an atmospheric boundary layer have been continuously controlled using a sodar, a temperature profile meter, and acoustic anemometers or thermometers mounted on a 56-meter-high mast. The joint action of a near-ground acoustic waveguide, the impedance of the underlying surface, and a vertical layered structure of the boundary atmospheric layer on a signal shape far from the acoustic source are studied.     

Space and time variations in the fine structure of the upper atmosphere according to acoustic sounding data

The results of studying variations in the fine layered structure of the upper atmosphere (heights of 20–140 km) according to data obtained from acoustic sounding within the range of infrasonic waves are given. The sources of infrasounds were surface explosions equivalent to 10 kg to 70 t of TNT. These explosions were set off in different seasons in different regions of Russia. Experimental data obtained in 1981–2011 have been analyzed. It has been found that the fine structure in the form of vertically distributed layered formations occurs in the upper atmosphere in all seasons. Moreover, the vertical distribution of both air-temperature and wind-velocity inhomogeneities in the upper atmosphere may be invariable over a time interval of no less than several hours. It has also been found that, throughout the entire atmospheric thickness from the stratopause to the lower thermosphere heights (up to 140 km), the instantaneous height distribution of layered air-temperature and wind-velocity inhomogeneities may remain almost unchanged during a time interval of no less than 20 min.     

Analysis of optimal conditions for recording signals when studying the atmospheric boundary layer with the acoustic method of partial reflection

Equations for the coefficient of partial reflection K from stratified inhomogeneities in the atmospheric boundary layer have been derived on the basis of the Epstein transition and symmetrical layer models as functions of three dimensionless parameters, i.e., the relative layer altitude, its relative thickness, and the relative variations in the effective sound speed in a layer. The equations have been obtained for the relative layer altitude at which the total internal reflection appears; the behavior of the function K is studied at close altitudes. Significant weakening of the dependence of coefficient K on the relative layer thickness in these conditions is shown, which makes it possible to record partially reflected signals in a wide range of wave-lengths or frequencies of the sounding signal. In other cases, the coefficient of partial reflection K strongly depends on the layer thickness. According to experimental data on variations in the amplitude of received acoustic signals with an increase in the source-detector distance, a technique for the parameterization of the additional impedance attenuation of sound that propagates over the earth’s surface has been developed, and these parameters have been experimentally estimated for different stratification conditions and sounding signal frequencies. Many records of background acoustic noises typical for one or another measurement sites have been distinguished and classified, a technique for estimating the minimum signal amplitude distinguishable against noises has been developed, and the corresponding estimates have been made. Based on these data and the specifications of three different industrial acoustic sources, the parameter limits provided by these sources have been estimated for the method of partial reflection.     

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.     

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.     

Experience in measuring the wind-velocity profile in an urban environment with a Doppler sodar

The experience of long-term acoustic remote measurements of vertical wind-velocity profiles at two sites in Moscow is reported. Equipment performances and measurement conditions are described. Acoustic measurement features characteristic of a large city with high traffic noise and spurious reflections from buildings are discussed. Criteria and techniques of rejecting noisy and false signals are described as well as the methods of statistical data processing suitable in the case of a signal-to-noise ratio rapidly varying in time and a significant number of rejected signals. Preliminary results of measurements are given.     

Simulation of additive transport from the land surface on the basis of experimental data on heat transfer in the atmospheric surface layer

A method is proposed to study the transport of a passive additive in the atmospheric surface layer with the use of the atmospheric transfer function. This method makes it possible to estimate the spatial distribution of the concentration of a passive additive in the atmospheric surface layer from the additive’s surface source without experimentally determining the vertical profile of the transport coefficient or without resorting to various hypotheses for the character of its behavior. The transfer function, which contains the information on the wind-field structure, can be obtained from simple one-point measurements of surface-and air-temperature fluctuations and from subsequent spectral processing of the data. The effects of the wind-velocity profile and turbulence on the spatial distribution of additive concentration are assessed. This method allows one to simplify experiments during development and verification of the models of atmospheric diffusion. This method may also be useful in emergency situations to predict the propagation of hazardous additives.     

Reflection response for normal incidence probing of vertically nonhomogeneous media

A time domain synthetic reflection seismogram is detailed and, as a limiting condition on this model, the analytic reflection impulse response for a one-dimensional lossless acoustic medium with piecewise continuous acoustic impedance is obtained. This analytic impulse response solution, in the structure of a sum of terms by order of reflection, provides insight to some of the poorly understood aspects of acoustic reflections from stratified and smoothly varying media as may be encountered in shallow marine sediments and elsewhere. It offers as well an approach for the inverse problem of extracting acoustic impedance profiles from reflection response data, though other effects (such as wavefront spreading, dispersive and absorptive attenuation, and wavelet broadening attendant with pulse propagation through a medium) need to be accommodated.     

Nonlinear effects manifested in infrasonic signals in the region of a geometric shadow

Nonlinear effects manifested in infrasonic signals passing through different atmospheric heights and recorded in the region of a geometric shadow have been studied. The source of infrasound was a surface explosion equivalent to 20–70 t of TNT. The frequencies of the spectral maxima of infrasonic signals, which correspond to the reflections of acoustic pulses from atmospheric inhomogeneities at different heights within the stratosphere-mesosphere-lower thermosphere layer, were calculated using the nonlinear-theory method. A satisfactory agreement between experimental and calculated data was obtained.     

Geostrophic flow disturbances generated by inhomogeneities of the gravitational field

A theoretical problem of linear stationary disturbances of the background geostrophic flow of a stratified rotating medium (atmosphere) that are induced by inhomogeneities of the gravitational field is considered. There is a common belief that such inhomogeneities may only somewhat deform (distort) the state of hydrostatic equilibrium, but cannot affect the dynamics of the flow in the atmosphere. Generally, the problem statement is different for the processes over a solid surface and a water surface, because a water surface (the lower boundary condition for the atmosphere) is deformed by inhomogeneities of the gravitational field. The problem of disturbances over a water surface has been considered in recent papers of the authors; in this paper, the results are developed and significantly revised. The emphasis is on disturbances over a flat horizontal surface, which were not examined before. From the analytical solutions, it follows that the influence of inhomogeneities of the gravitational field on the atmospheric flows may be significant in some cases. Physical generation mechanisms of disturbances are analyzed.     

完全非线性孤立波的直墙反射

报道了应用边界积分方法模拟完全非线性孤立波的传播与直墙反射,给出了波形演变过程。结果表明,本模型对计算孤立波的传播与直墙反射是有效的。三阶Boussinesq方程的孤立波解比低阶方程的孤立波解更接近完全非线性的数值解.当来波波高增大时,孤立波直墙反射的相位滞后变小。若考虑大波高孤立波的直墙反射或波——波相互作用,一阶理论预报的相位滞后往往低估实际情况。     

An investigation on internal solitary waves in a two-layer fluid: Propagation and reflection from steep slopes

Experimental investigations on internal solitary wave (ISW) propagation and their reflection from a smooth uniform slope were conducted in a two-layered fluid system with a free surface. A 12-meter-long wave flume was in use which incorporated with: (1) a movable vertical gate for generating ISW; (2) six ultrasonic probes for measuring the fluctuation of an ISW; and (3) a steep uniform slope (from one of θ=30°, 50°, 60°, 90°, 120° and 130°) much greater than those ever published in the literature. This paper presents the wave profile properties of the ISW recorded in the flume and their nonlinear features in comparison with the existing Korteweg de Vries (KdV) and modified Korteweg-de Vries (MKdV) theories. Experimental results show that the KdV theory is suitable for most small-amplituded ISWs and MKdV theory is appropriate for the reflected ISWs from various uniform slopes. In addition, both the amplitude-based reflection coefficient and reflected energy approach a constant value asymptotically when plotted against the slope and the characteristic length ratio, respectively. The reflected wave amplitudes calculated from experimental data agree well with those reported elsewhere. The optimum reflection coefficient is found within the limit of 0.85 for wave amplitude, among the test runs from steep normal slope of 30° to inverse angle of 130°, and around 0.75 for the reflected wave energy, produced by an ISW on a vertical wall.     

Horizontal and oblique spectra of temperature fluctuations in a stably stratified troposphere

The first experimental studies of the spatial oblique and vertical spectra of temperature fluctuations in a stably stratified troposphere at heights of 2 to 8 km were conducted. The measurements were taken over northern European Russia. The spectra cover the wave number range from 5 10^?4 to 3 10^?2 rad/m. The estimates obtained for the spectral density are analyzed on the basis of a model developed previously for the three-dimensional (3D) spectrum of temperature fluctuations generated by a statistical ensemble of internal waves. This model made it possible to consider both oblique and horizontal spectra from a unified point of view and to use a unified set of parameters on the basis of the 3D spectrum concept. The quantitative estimates obtained for the parameters of the 3D spectrum have shown that large-scale temperature inhomogeneities with a vertical size of more than a hundred meters are strongly extended along the land surface. They have approximately the same form; their horizontal sizes are at least 20 times greater than their vertical sizes. The anisotropy of temperature inhomogeneities decreases with a decrease in their vertical sizes and reaches 1.5–2 for vertical sizes of 10–20 m or smaller.     

Coherent reflection of an acoustic plane wave from a random sediment layer over an elastic sea floor

The problem of coherent reflection of an acoustic plane wave from a seabed consisting of a randomly inhomogeneous sediment layer overlying a uniform elastic sea floor is considered in this analysis. The random perturbation in the sediment layer is attributable to the sound-speed variations, resulting in volume scattering due to medium inhomogeneities. An approach based upon perturbation theory, combining with a derived Green's function for a slab bounded above and below, respectively, by a fluid and an elastic half-space, is employed to obtain an analytic solution for the coherent field in the sediment layer. A linear system is then constructed to facilitate the computation of the coherent reflection field. The results of the coherent reflection coefficient for various sediment randomness, frequency, sediment thickness, and sea floor elasticity have been numerically generated and analyzed. It is found that the higher/larger the randomness, frequency, thickness, and shear-wave speed, the lower is the coherent reflection. Physical interpretations for the characteristics of the various results are provided.     

Numerical modeling of the propagation of nonlinear acoustic-gravity waves in the middle and upper atmosphere

A numerical algorithm for modeling the vertical propagation and breaking of nonlinear acoustic-gravity waves (AGWs) from the Earth’s surface to the upper atmosphere is described in brief. Monochromatic variations in the vertical velocity at the Earth’s surface are used as an AGW source in the model. The algorithm for solving atmospheric hydrodynamic equations is based on three-dimensional finite-difference analogues of fundamental conservation laws. This approach selects physically correct generalized solutions to hydrodynamic equations. A numerical simulation is carried out in an altitude region from the Earth’s surface to 500 km. Vertical profiles of the background temperature, density, and coefficients of molecular viscosity and heat conduction are taken from the standard atmosphere models. Calculations are made for different amplitudes of lower-boundary wave forcing. The AGW amplitudes increase with altitude, and waves may break in the middle and upper atmosphere.     

Lidar sounding of turbulence based on the backscatter enhancement effect

A simple derivation of the equations describing the backscatter enhancement (BSE) effect of waves on small inhomogeneities in a randomly inhomogeneous medium is presented. The BSE effect is considered in a locally isotropic turbulent atmosphere. It is shown that a system of remote sounding of atmospheric turbulence can be constructed on the basis of BSE measurements. The scheme of a lidar for BSE measurement, along with routine lidar sounding, is proposed. With the use of models it is shown that regions of increased turbulence can be detected with such a lidar.     

Consideration of fine-scale coastal oceanography and 3-D acoustics effects for the ESME sound exposure model

Results and recommendations for evaluating the effects of fine-scale oceanographic scattering and three-dimensional (3-D) acoustic propagation variability on the Effects of Sound on the Marine Environment (ESME) acoustic exposure model are presented. Pertinent acoustic scattering theory is briefly reviewed and ocean sound-speed fluctuation models are discussed. Particular attention is given to the nonlinear and linear components of the ocean internal wave field as a source of sound-speed inhomogeneities. Sound scattering through the mainly isotropic linear internal wave field is presented and new results relating to acoustic scattering by the nonlinear internal wave field in both along and across internal wave wavefront orientations are examined. In many cases, there are noteworthy fine-scale induced intensity biases and fluctuations of order 5-20 dB.