This paper it is shown that for an effective study of cirrus clouds by the method of laser polarization sensing, it is necessary to have: at least two wavelengths (for example, 0.532 and 1.064 μm); the ability to construct a color ratio for this pair of wavelengths; and the ability to measure the lidar and depolarization ratios at any from wavelengths. The use of a wavelength with strong absorption for ice (for example, 1.55 or 2.15 μm) is demonstrated to be promising for retrieving the average particle size in the cloud. Therefore, the combined use backscatter ratios measured by lidar (color, lidar, and depolarization ratios) demonstrates its effectiveness both in retrieving the shape and size of cirrus cloud particles.
Lidar systems are an effective tool for studying the optical and microphysical properties of scattering media. The efficiency of interpretation of lidar sensing, in turn, depends on the quality of processing measurement data. One of the severe difficulties in solving these problems is that the propagation of laser beam in optically dense scattering media is accompanied by the appearance of a multiple scattering background, which can be estimated by solving the nonstationary transfer equation. However, in the case of the presence of cirrus clouds on the sounding path, data analysis is carried out mainly in the single scattering approximation. The paper considers the features of the propagation of laser pulse generated by a space-borne monostatic lidar in two-layered cirrostratus cloudiness, taking into account multiple scattering. The temporal structure of the echo signals obtained using the original algorithm of the Monte Carlo method is analyzed. It is shown that the ratio of the power of the multiple-scattered lidar signal to the power of the single-scattered signal in the subcloud spaces of the Rayleigh atmosphere can be higher than within the cloud.
The paper presents a comparison of numerical solutions for the problem of light scattering on ice particles using methods of geometric and physical optics. The solutions were developed for irregularly shaped particles ranging in size from 10 to 100 μm, with a wavelength of 0.532 μm. The results of the comparison of the light scattering matrix over the entire direction sphere indicate that the solution within the framework of geometric optics approximation is in good agreement with the solution within the framework of physical optics over the entire scattering sphere, except for the vicinity of scattering in the forward and backward directions.
The paper presents the results of comparing numerical solutions to the problem of light scattering on atmospheric ice particles by methods of geometric and physical optics. The solution was constructed for hexagonal shaped particles (hexagonal ice column) and droxtals. The results of the comparison of the light scattering matrix over the entire sphere of the direction showed that in the direction of scattering strictly backward, the use of the geometric optics approximation can lead to significant errors, especially for hexagonal particles, which are characterized by the presence of angular reflection
The paper presents a solution to the problem of light scattering by small randomly oriented hexagonal ice crystals of cirrus clouds. The solution was obtained both within the discrete dipole approximation and within the physical optics approximation. The particle sizes ranged from 0.2 to 6 μm. The wavelength of the incident radiation is 0.532 μm, the refractive index is 1.3116. The results of the comparison of the solutions obtained within the framework of the discrete dipole approximation with the physical optics approximation are shown. It is found that the solutions are in good agreement, however, to solve the problem of light scattering in the vicinity of the backward direction, which is important for the interpretation of lidar data, it is necessary to continue the calculations by the discrete dipole approximation up to particle sizes of about 10 μm.
The work proposes an original algorithm of the Monte Carlo method for simulating the propagation of laser pulses in a cloudy atmosphere taking into account all scattering orders. The results of studying the features of echo signal propagation through the cloud represented by semi-infinity homogeneous layer are discussed. The contribution of multiple scattering to a ground-based and a spaceborne monostatic lidar returns from clear atmosphere obscured by cumulus, stratus or cirrus clouds is assessed at the wavelength of 0.532 μm. The significant presence of multiple scattering in lidar returns and its dependence on the characteristics of the sounded clouds are demonstrated. High anisotropy of scattering in the forward direct ion by polydispersions of large nonspherical ice crystals is the basis of a tangible effect of multiple scattering for small values of the lidar receiver field of view. The radiation scattered by the crystal particles within a narrow cone around the forward direct ion is predominant and is greater than the corresponding values for scattering on water droplets. For cumulus and stratus clouds, the contribution of triple-scattered photons and higher to the total echo signal increases with increasing field of view of the receiver. The magnitude of multiple scattering effects is also seen to be largely determined by the optical depth across the receiver field of view at the cloud.
The optical characteristics of arbitrary shaped atmospheric ice crystals are necessary both for solving the problem of radiation transfer in the atmosphere and for interpreting lidar data. At the same time, there is an acute problem of studying the influence of the particle shape on its optical characteristics. Since the construction of a databank of the optical characteristics of cirrus clouds is a laborious task, it is necessary to reduce as much as possible the number of different particles, the superposition of which represent the values averaged over the entire ensemble of particles in the cloud. The report presents the results of calculations of the light scattering matrix for 500 different arbitrary shaped particles with 25 facets and the same particle size. The calculations were performed within the geometric optics approximation. According to the results of the analysis, several particles were selected whose optical properties are as close as possible to the optical properties of the entire ensemble.
The purpose of this work is to solve an important issue: the light scattering problem for ice crystals of cirrus clouds less than 10 μm and matching the obtained solution with the existing solution obtained within the physical optics approximation. The article presents a solution to the problem of light scattering by hexagonal ice particles of cirrus clouds with sizes from 0.05 to 5.17 μm for a wavelength 0.532 μm, obtained within the discrete dipole approximation. It is found that the obtained solution is in good agreement with the physical optics approximation in the vicinity of scattering angles of 0–10° (the vicinity of forward direction scattering). However, to solve the problem of light scattering in the vicinity of the backward scattering direction, which is important for the interpretation of lidar data, it is necessary to continue the calculations to sizes of the order of 20 μm. The results obtained are necessary for constructing algorithms for the interpretation of lidar data obtained by sounding cirrus clouds.
The paper presents a solution to the problem of light scattering by hexagonal atmospheric plates and columns, as well as irregularly shaped particles with sizes from 10 to 100 microns. The solution is presented in the form of a databank of light backscattering matrices. The solution was obtained for typical wavelengths used in laser sensing problems: 0.355, 0.532, 1.064 μm; as well as for the wavelengths of the near infrared range: 1.55, 2 and 2.15 μm. At wavelengths of 0.532 and 1.064 μm, in addition to the refractive index of ice, the refractive index of the dust aerosol was used: 1.48+i•0.002 and 1.6+i•0.002, respectively. The solution was obtained within the framework of the physical optics method developed by the authors. Based on the calculated light backscattering matrices, the values of the color and linear depolarization ratios were obtained. It is shown that the power laws previously identified by the authors are violated in the presence of absorption, in particular, for hexagonal particles with sizes up to 100μm, with an imaginary part of the refractive index greater than i•0.0004, significant deviations from the power law are observed. For irregularly shaped particles at wavelengths for which there is no absorption, smooth power law dependences are seen.
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