In the geometrical optics approximation, the scattering of solar radiation in the visible and near-IR regions of the solar spectrum by particles of crystalline clouds is numerically studied. For this purpose an algorithm for direct statistical simulation has been developed. A feature of the algorithm is the ability to obtain the scattering characteristics of solar radiation by crystalline particles of various shapes, concentrations and spatial orientations in a single calculation. Another feature is that, along with particles having smooth surfaces, the processes of radiation scattering on particles with rough surfaces are simulated taking into account the effects of shading and re-reflection. The algorithm is based on the so-called “facet” model of randomly inhomogeneous surfaces. A series of numerical experiments has been performed.
KEYWORDS: Optical transfer functions, Scattering, Light scattering, Reflection, Monte Carlo methods, Refraction, Fourier transforms, Absorption, Water, Signal attenuation
In the paper, we present an algorithm of the Monte Carlo method for calculating the optical transfer function (OTF) of the ocean-atmosphere system based on a combination of the methods of “correlated sampling” and “conjugate trajectories”. Some numerical estimates of the effect of a number of factors on the OTF system are presented.
A stochastic non-Gaussian model of the random field of the attenuation coefficient in stratus cloudiness is presented, as well as a weighted Monte Carlo algorithm for simulating the optical radiation transfer in a random cloudy medium.
We consider the problem of optical radiation transfer in the ocean-atmosphere system in the beam optics approximation for the complete and simplified “facet” models of the random agitated ocean surface. Various aspects of Monte-Carlo numerical solutions for certain direct problems of passive and active aerospace optical sensing of the ocean are discussed. The considered models and algorithms may be useful in radiation field analysis in the oceanatmosphere system by other numerical and analytical methods concerning the transfer equation.
Numerical data about the sensitivity of the echo-signal to variations in aerosol scattering coefficient define, as is well known, the possibility of solving the inverse problem of atmosphere laser sensing. In this work we analyze the influence of statistic fluctuations of the vertical distribution of the aerosol scattering coefficient on the time distribution of the LIDAR echo-signal in the case of a cloudless atmosphere. We obtain time distributions of intensity, averaged over random instances of aerosol scattering coefficient fields and the corresponding correlation coefficients.
The interaction of radiation of the Novosibirsk Free Electron Laser (FEL) at a wavelength of 130 μm in the atmospheric transmission window with a model aerosol cloud having the known droplet size distribution function has been studied experimentally. The experimental findings are compared with theoretical calculations obtained from solution of the lidar equation for the conditions of the experiment.
The results of numerical simulation of polarization characteristics of terahertz signals from a ground-based remote sensing system in stratus clouds for various models of liquid-droplet clouds are compared. Models of the scattering medium take into account the vertical stratification of the water vapor concentration in the atmosphere. The model of droplet size distribution includes droplets larger than 20 μm in radius. They are referred to as large droplets, while droplets with radius 1<r<20 μm are referred to as small ones. The model of a cloud layer was developed with allowance for a difference in the vertical stratification of the mean radii of the small and large droplets.
The paper presents results of numerical statistical simulations of experiments of ground-based sensing of cloud layers by terahertz linearly polarized radiation for certain wavelengths from the atmospheric transparency windows. Summarized results of many years’ field measurements of liquid droplet size distributions in temperate latitudes of the Earth and the distributions obtained by aircraft experiments off Great Britain’s coast are used in the scattering layer models. The models of the scattering medium take into account the vertical stratification of water vapor concentration in the atmosphere and the differences in cloud layer microstructure at the top and the base.
Time distributions of ground-based LIDAR echo-signals reflected by the bottom of a liquid droplets cloud were calculated with the help of local estimates of Monte Carlo methods for wavelengths from visible to submillimeter range. The calculations are performed for three models of clouds. In the first model the cloud layer consists only of small droplets of radius from 1 to 20 microns, in the second one there are small and large droplets of radius from 1 to 85 microns and in the third one we take into account small, large and supersize droplets. A density of a particle size distribution is generalized on results of cloud microstructure observations in temperate latitudes. Wavelength range for which one should take into account large and supersize droplets in cloud models is found.
The light haze of a ground-based impulse LIDAR in an altitude-wise optically inhomogeneous cloud-free atmosphere was computed in dependence of various optical-geometrical parameters of the experiment using the Monte Carlo method. Modeling of the LIDAR echo-signal reflected off the lower boundary of a liquid droplet cloud for a ground-based LIDAR and off the upper boundary of the cloud for an aircraft- and space-based LIDAR was also completed. A feature of these calculations is the choice of the model with altitude-wise statistically inhomogeneous structure of aerosol scattering parameters as the optical model of a cloudless atmosphere. The echo-signals from clouds were computed in the assumption of statistical variation of the altitudes of the lower and upper cloudiness boundaries. In the calculations new effective discrete-stochastic modifications of local estimates were employed.
Laser sensing is an effective way of studying optical properties of various atmospheric structures. If we consider
strongly scattering media, like clouds, there arises the necessity of taking into account the effects of multiple
scattering which changes the space and time characteristics of the light pulse. The Monte Carlo method is the
most convenient one for obtaining practical results in such problems. In this paper two problems were solved.
One is constructing an adequate optical model of crystal clouds taking into account optical anisotropy of the
medium. The other is Monte Carlo modeling of laser radiation transfer in such a medium. The form and duration
of light pulses reflected by clouds (lidar returns) are obtained by the Monte Carlo method in the case of single
layer continuous crystal cloud and double layer continuous cloudiness (a crystal cloud of highest level is located
above a drop cloud).
Laser sensing is an effective way of studying optical properties of various atmospheric structures. If we consider
strongly scattering media, like clouds, there arises the necessity of taking into account the effects of multiple
scattering which changes the space and time characteristics of the light pulse. The Monte Carlo method is the
most convenient one for obtaining practical results in such problems. In this paper two problems were solved.
One is constructing an adequate optical model of crystal clouds taking into account optical anisotropy of the
medium. The other is Monte Carlo modeling of laser radiation transfer in such a medium. The form and duration
of light pulses reflected by clouds (lidar returns) are obtained by the Monte Carlo method in the case of single
layer continuous crystal cloud and double layer continuous cloudiness (a crystal cloud of highest level is located
above a drop cloud).
The present paper is devoted to modeling radiative heat transfer in forming planetary systems. A solution of the transfer equation with multiple sources for dusty media of spherical polydispersive particles is considered. An algorithm for computation of the temperature distribution in circumstellar disks is proposed. Coincidence of numerical results and known analytical cases is shown. The spatial distributions of optical characteristics and matter
temperature of β - Pictoris debris disk are obtained for a low spatial resolution.
The problems of statistical simulation of light pulse propagation in stochastic scattering media as applied to the problems of laser monitoring of the clouds are considered. A set of Monte Carlo algorithms, allowing the construction of numerical models for the field of multiply scattered narrow light beams in the aerosol atmosphere and continuous cloudiness has been provided for the purpose. A special attention has been paid to solving the problem of optimization of Monte Carlo algorithms. The optimization is based on the method of "dependent trials."
The present paper considers a set of problems of statistical modeling for the transport of solar radiation in stochastic natural media as applied to the problems of remote sensing of the ocean and aerosol and cloudy atmosphere as well as to the problems of constructing the numerical models of solar radiation fields in clouds and cloudy atmosphere. The basis of this consideration is the statistical approach when the radiation transfer is described by the equation, in which some of the parameters are random functions. A set of new Monte Carlo algorithms and programs has been provided for the purpose. This set includes algorithms for the simulation of homogeneous stationary random fields of continuous and broken cloudiness, the optic parameters of the atmosphere and the ruffled sea surface as well as the simulating the radiation transfer process in stochastic media. A special attention has been paid to solving the problem of optimization of Monte Carlo algorithms.
KEYWORDS: Databases, Data modeling, Data storage, Modeling, Human-machine interfaces, Atmospheric modeling, Monte Carlo methods, Photonics systems, Telecommunications, Data communications
In the work, the automated system `Photon' for statistical modeling of a radiation field of the atmosphere is presented. The scheme of functional construction of the system, a list of problems solved by the system and the database contents are given.
A numerical model of the wind-driven undulation of the sea surface is constructed in this paper. This model was used for studying optical properties of the sea surface by the Monte Carlo method. It is shown that simulation of the field of the sea surface elevations makes it possible to more precisely determine the optical properties of the swell with allowance for multiple reflection effects and radiation shading by surface elements.
KEYWORDS: Stochastic processes, Optical transfer functions, Scattering, 3D modeling, Atmospheric modeling, Signal attenuation, Clouds, Monte Carlo methods, Reflection, Scattering media
The paper presents two algorithms of the Monte Carlo method for estimating the optical transfer function (OTF) of the atmosphere-underlying surface system. Some results of numerical calculations of the mean value of the frequency- contrast characteristics of the scattering coefficients of a random field are presented. The calculations are based on horizontally-inhomogeneous models of the stochastic atmosphere, built on the Palms fluxes. The values, on the average, increase when transferring from the determined model to a one-dimensional stochastic model and then to a 3- D stochastic model. The obtained values of the frequency- contrast characteristics (FCC) were used for the correction of the simulated 'space snapshots' of test objects.
Intensity fluctuations of laser beams propagating through precipitation reveal the regularities that are different from those
for the turbulent atmosphere. Specifically, three regimes of fluctuations can be observed. The first one is the weak
scintillation regime where the scintillation index increases with the propagation path. The second regime is the fluctuation
saturation where the scintillation index approaches a constant. This saturation level can be an arbitrary number unlike the
turbulent atmosphere. The further increase of the propagation path leads to damping of intensity fluctuations. This regime
has no analogy in the case of the turbulent atmosphere. In the proposed paper, the regime of fluctuation damping in
snowfalls is studied both experimentally and theoretically. The experimental setup consists of a narrow laser beam
propagating through a 2 km path. A small detector of 0.1 mm of diameter was placed on the optical axis. The dependence
of scintillation index on the optical depth of the path and of flowsnake sizes were considered. According to the fluctuation
model proposed early by the authors, the scintillation index can be calculated theoretically by means of numerical solution
of the radiative transfer equation. The equation is solved by the Monte-Carlo code with some model phase function for a
snowflake. The damping factor of scintillations is calculated as a function of the optical depth and of the angular divergence
of the laser beam. Comparison of the calculated and experimental data is presented.
Multiple scattering of laser beams in scattering media can be easy expressed through Greens function of
the radiative transfer equation. Therefore numerical calculations of Greens functions are more advisable in
comparison with calculations of optical signal for some concrete experimental schemes. In this paper,
Green's function for snowfalls is calculated with the standard Monte -Carlo code. A simplest model for
the phase function of a snowflake is used. The angular and spatial divergence of radiation due to multiple
scattering versus optical depths and particle sizes in snowfalls are discussed.
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