A. S. Gurvich

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.

Enhanced backscattering from a plane mirror viewed through a turbulent phase screen

The distribution of average intensity scattered by a plane mirror placed behind a turbulent phase screen is experimentally investigated for a point source of light. It was found that the average intensity distribution in the source plane coincides with the correlation function of intensity fluctuations of a virtual source located at the mirror and observed from the real source plane. Nonmonotonic dependence between the enhancement factor and the diameter of the mirror is observed in experiments. Experimental results are in good agreement with theoretical predictions.

Model of the three-dimensional spectrum of anisotropic temperature irregularities in a stably stratified atmosphere

A phenomenological model is proposed for the three-dimensional (3D) spectrum of temperature irregularities generated by internal waves in the atmosphere. This model develops a theory (Chunchuzov, 2002) based on the assumption that the field of the Lagrange displacements of the medium’s particles that are caused by a statistical ensemble of internal waves with randomly independent amplitudes and phases is stationary, homogeneous, axially symmetric in a horizontal plane, and Gaussian. To fit the model to measured spectra of fluctuations in the stratosphere and mesosphere, an additional assumption is introduced into the model that the degree of anisotropy of irregularities depends on their vertical size. An explicit expression is presented for the 3D spectrum. The model vertical spectrum follows a power law with an exponent of −3. The horizontal spectrum has three asymptotically power portions. Two of these are characterized by an exponent of −3, whereas an intermediate portion has an exponent of −1 to −3, depending on the rate at which the degree of anisotropy decreases as the vertical size of temperature irregularities increases. Simple asymptotic formulas are obtained for the horizontal spectrum. Within the range of a few decades, the model is in good agreement with the published results of measuring the spectra in the upper troposphere, stratosphere, and mesosphere.

Determination of parameters of the spectrum of internal waves in the stratosphere from space-based observations of strong stellar scintillation

Observation of stars from a spacecraft through the Earth’s atmosphere is a constituent part of remote sensing of the atmosphere. Recorded scintillation signals contain data on the structure of air-density irregularities induced by turbulence and internal waves. Currently, parameters of the structure in the stratosphere are determined using the procedures based on the weak-scintillation theory. However, during stellar occultation by the Earth’s atmosphere, scintillation becomes stronger as the line of sight plunges into denser air layers. This paper considers the problem of remote sensing of stratospheric irregularities under strong-scintillation conditions. The scintillation spectra are calculated in the phase-screen approximation under the assumption that the spectrum of the phase added by the screen corresponds to observations through the stratosphere. It is assumed that stratospheric irregularities of air density are generated by an ensemble of saturated internal waves whose three-dimensional spectrum contains two characteristic wave numbers corresponding to the outer and inner scales. In the calculation, no restrictions are imposed on the observed scintillation amplitude. It is shown that the effect of the scintillation amplitude on the observed scintillation spectra appears most prominent for large wave numbers corresponding to irregularities whose sizes are smaller than the inner scale. For these wave numbers, deviations from the weak-scintillation theory become appreciable if the rms relative fluctuation of light intensity exceeds 0.3. In contrast, for small wave numbers corresponding to scales exceeding the outer scale, the weak-scintillation theory remains valid to rms values as large as 2. Analysis of calculated spectra has shown that the parameters of the three-dimensional spectra of stratospheric irregularities can be retrieved under the conditions of relatively strong scintillation characterized by an rms value below 1.5–1.6.

Lidar positioning of higher clear-air turbulence regions

The possibility of lidar positioning of regions with higher clear-air turbulence (CAT) is shown. The turbulence is indicated by air density fluctuations generated by it. A scheme with a lidar based on using the backscattering enhecement (BSE) effect in a turbulent medium is considered. A stable solution of the positioning problem is obtained using the statistical regularization method. As is shown on models, CAT regions that are dangerous for civil aviation flights can be detected using such a lidar.

Strong scintillation spectra behind the atmosphere with large- and small-scale inhomogeneities

The numerical examination of 2D spectra of strong stellar scintillations observed through the Earth’s atmosphere from space is carried out. The atmosphere contains a combination of statistically independent, anisotropic large-scale and isotropic small-scale inhomogeneities of the refractive index. 2D spectra and vertical and horizontal 1D spectra calculated using them are presented. It is shown that the strong scintillation spectra are not equal to the sum of the spectra formed by separate, statistically independent components. The combination of large- and small-scale inhomogeneities results in a greater dispersion of scintillations in comparison with the absence of latter ones. However, the presence of this combination can lead both to an increase and decrease of dispersion in comparison with the sum of dispersions of the anisotropic and isotropic components depending on their intensity relation. The new effect in 1D horizontal spectra behavior is found in the region of small wave numbers; i.e., the presence of small-scale atmospheric inhomogeneities results in the suppression of spectral power of scintillation formed by only an anisotropic component.

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.

Comparison between refraction angles measured in the Microlab-1 experiment and calculated on the basis of an atmospheric general circulation model

The differences between the refraction angles measured and calculated for the reanalyses of the European Centre for Medium-Range Weather Forecasts were statistically analyzed on the basis of 64 radio occultation events recorded by the Microlab-1 satellite. It is shown that, for minimum ray heights below 20 km, the main contribution to the differences is made by spatial refractive-index fluctuations neglected by the model. The power spectral density of these fluctuations is mainly concentrated within the vertical wave-number range 0.5–10 rad/km. For heights above 30 km, the deviations are mainly determined by ionospheric disturbances and may vary several times during changes of the site and time of observations. This suggests that the results of satellite radio-occultation sounding of the neutral atmosphere can be used as an indirect quantitative estimate of local discrepancies between the actual field of the refractive index and its values calculated on the basis of a hydrodynamic atmospheric general circulation model.

Thermal blooming of laser beams in turbulent media

This paper presents the results of laboratory experiments that studied the joint effect of turbulence and thermal blooming on laser beam propagation. The effect of turbulence on thermal defocusing, changes in the intensity fluctuations, and the coherence of radiation were investigated under the conditions of the self-action of narrow and broad beams both in the cw and pulse regimes of laser generation.

Lidar sounding of the optical parameter of atmospheric turbulence

The operation of a lidar intended for clear air turbulence (CAT) positioning on the basis of the backscatter enhancement (BSE) effect is analyzed using a turbulence model with a power-law spectrum. Systematic distortions occurring due to a need to regularize the lidar positioning problem solution are estimated. It is shown that the effect of molecular viscosity of air on the positioning result can be neglected if the wave parameter, which characterizes the diffraction manifestation, is higher than 3. This corresponds to sounding ranges of more than 1 km for optical or UV lidars. The analysis results show that the BSE lidar positioning accuracy weakly depends on the exponent in the turbulence spectrum in regions of severe turbulence. The results can justify a physical experiment for the design of an aircraft system for the lidar detection of CAT regions ahead of the flight course.