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.
The currently accepted algorithm for retrieving the optical parameters of clouds from satellite-based radiance measurement data implies the application of independent pixel approximation and a model of uniform cloudiness. This single-pixel approach cannot capture the 3D effects manifest themselves across multiple satellite pixels. At the same time, three-dimensional radiative transfer effects are the major source of retrieval errors in the passive remote sensing of clouds. The paper describes an alternative approach that extends the capabilities of traditional methods to obtain the estimates of cloud parameters. It is based on the building artificial neural network trained with the simulation data of solar radiative transfer in the cloudy atmosphere. The possibility of retrieving the optical thickness of broken horizontally inhomogeneous warm clouds is demonstrated. High values of the correlation coefficient between the benchmark and retrieved values of the optical depth were achieved at a fixed effective radius of cloud drops. The dependence of the correlation coefficient on the hyperparameters of the neural network, the volume and structure of the training dataset is studied.
Previously, we built a database for a single particle with a random polyhedral shape. The solution was obtained for a size range from of 10 to 300 microns at a wavelength of 0.532 microns. The particle selected was the one closest to the average in the ensemble of generated particles. In this work, a different approach to building the database is implemented: an arbitrary particle with a random shape and size is generated from the range of particle sizes attached to the database. The report examines the differences between the two approaches in building the database.
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
A technique is proposed that makes it possible to significantly accelerate the solution of the problem of light scattering in the exact backscattering direction on particles larger than 100 microns within the framework of the physical optics method. The peculiarities of this technique is that the solution is constructed within the vicinity of a small number of nodal points. Then, in the resulting solution, one complete period of interference oscillations is allocated and the light scattering matrix is averaged in this range. The average value is entered into the database. In this way, it was possible to build a complete database of backscattering matrices for atmospheric ice particles with sizes from 10 to 1000 microns for wavelengths of 0.355, 0.532, 1.064, 1.55, 2 and 2.15 microns. The results obtained make it possible to improve the algorithms for interpreting the data of laser sensing of cirrus clouds.
The current algorithm for retrieval of the optical and microphysical properties of clouds from satellite measurement data involves the use of independent pixel approximation and a model of plane-parallel horizontally and vertically homogeneous clouds for each pixel of satellite image, that in some situations can lead to large retrieval errors. The paper describes an alternative approach based on applying neural networks trained using the results of 3D simulation of solar radiation transfer in the Earth’s atmosphere in the presence of a field of broken horizontally inhomogeneous clouds. To generate a cloud field realization, the fractal multiplicative bounded cascade model is used. The examples of modeling the stratocumulus clouds by means of the bounded cascade model are considered. The model is successfully tested using the MODIS image of stratocumulus cloud scene.
This paper presents the results of calculations and analyzes the backscattering matrices of random oriented ice crystals, typical for cirrus clouds. The backscattering matrices were calculated within the physical optics approximation for lidar (wavelength 0.355 μm) and the discrete dipole approximation for 94 GHz radar (wavelength 3189 μm) with the refractive indices of ice – 1.3249+0i and 1.7864+0.0032i respectively. For the first time, it was possible to correctly calculate the ratio of radar and lidar signals in the direction of backscatter, called the radar-lidar ratio. The radar-lidar ratio is obtained for a wide particle size distribution for typical ice crystal shapes in cirrus clouds: hexagonal column and plate, bullet, droxtal, aggregate, bullet-rosette, arbitrary shape and sphere. As a result, it is shown that the radar-lidar ratio can be used to estimate the ice crystals size in cirrus clouds. The radar-lidar ratio for the EarthCARE satellite was unambiguously obtained for an adequate model of the sizes and shapes of ice crystals of a cirrus cloud.
The paper presents the light backscattering matrices for crystal particle of an irregular convex shape of the size of 200 micrometers with different refractive indices within the physical optics approximation. The calculation of matrices was carried out at the wavelengths of incident light of 0.532 micrometers, for the case of an arbitrary spatial orientation of particles. The matrices can be used to study mineral dust cloud particles.
Scattering of light by ice crystals of cirrus clouds is an important problem for remote sensing of clouds and the atmosphere. Such a solution is necessary for the interpretation of data obtained be the active and passive remote sensing instruments, such as lidars and photometers. Ice crystals in cirrus clouds are large non-spherical particles. To solve the problem of light scattering by large non-spherical particles, it is not possible to apply rigorous numerical methods such as FDTD, DDA, PSDT, due to their high computational complexity for large particles. This problem also cannot be solved within the framework of the classical geometric optics approximation, since this method does not take into account such phenomena as diffraction and interference, which have a significant effect in the vicinity of the backscattering direction. This report presents the solution for the problem of light scattering by non-spherical particles with sizes from 10 to 1000 μm. The results were obtained within the framework of the physical optics approximation. The obtained database of light scattering matrices allows one to improve the accuracy of interpretation of laser sounding data of atmospheric aerosol and crystal clouds for lidars and remote sensing applications.
The paper presents a solution to the problem of light scattering by randomly oriented particles of irregular shape for particles with sizes of 100, 140, 170 and 200 microns for a wavelength of 0.532 microns for various refractive indices. The solution was constructed both within the framework of physical optics (for the backward scattering direction) and geometric optics (for scattering angles in the range from 0 to 180 degrees). The obtained solutions made it possible to construct a diagram of the dependence of the geometric albedo of a particle on the maximum degree of polarization to verify the Umov effect. It was found that when the imaginary part of the refractive index is less than 0.001, the Umov effect is performed with good accuracy. However, for the case when the imaginary part of the refractive index is greater than 0.001 and the specular component of the scattered radiation begins to dominate in the solution, the Umov effect is violated.
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 optical characteristics of atmospheric ice particles are usually calculated within the framework of the physical optics approximation, since particle sizes generally vary from 10 to 1000 microns. However, the results of experimental measurements show that ice crystals up to 10 microns in size are observed in cirrus clouds of the upper tier. The report presents a solution to the problem of light scattering for particles, obtained in the framework of the methods of the physical optics and discrete dipoles. Based on the solution, such important optical characteristics as depolarization and spectral ratios were calculated. Two limiting cases are considered: ideal hexagonal particles and randomly shaped particles.
The solution to the problem of light scattering by atmospheric hexagonal plates and columns, as well as arbitrary shaped particles with sizes from 10 to 100 microns, is presented. The solution is presented as a databank of light backscattering matrices and is intended for interpretation of data from ground-based and space-based lidars. The solution is obtained for typical wavelengths used in laser sensing problems: 0.355; 0.532; 1.064; as well as for the wavelengths of the near infrared range: 1.55; 2 and 2.15 μm. The solution was obtained within the framework of the physical optics method.
When solving the problem of light scattering on ice crystals of cirrus clouds, two limiting cases can be distinguished: a model of ideal hexagonal particles and a model of randomly shaped particles. As already shown, the optical characteristics of particles that are closer to the real forms of atmospheric crystals are not go beyond these limiting cases. Before solving the problem of light scattering within the framework of the physical optics method, it is necessary to carry out a number of basic estimates. These estimates include the study of the convergence of the numerical solution on the number of iterations of the beam splitting algorithm and the number of spatial orientations of the particle during its averaging. The results of this study are presented in the report.
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.
The Umov effect is the inverse correlation between the maximum of the linear polarization of the light scattered on an object and the geometrically albedo of this object. The importance of studying this effect should be considered in the context of one of the complex and important tasks of remote sensing: determining the concentration of particles in optically thin clouds. Since the intensity of the scattered light depends on two unknown quantities: the concentration of particles in the cloud and the phase function of the particles of the cloud. Then to retrieve the concentration from the measured signal, it is necessary to know the phase function in advance. In real observations, the phase function is, usually, not known. The Umov effect will make it possible to estimate some necessary unknown characteristics of particles in a cloud, which determine the phase function. This paper is devoted to the study of this effect for particles with sizes much larger than the wavelength of the incident light. The report presents a solution to the problem of light scattering by randomly oriented particles of irregular shape for particles with sizes of 100 and 200 microns, for a wavelength of 0.532 microns. The solution was obtained within the both frameworks: the physical optics method and the geometric optics approximation. It was found that if the imaginary part of the refractive index less than 0.001, the Umov effect is observed with good accuracy for particles of size from 100 to 200 microns. However, the Umov effect is violated when the imaginary part of the refractive index is greater than 0.001.
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