Studying the Polarization Characteristics of Thermal Microwave Emission from a Cloudy Atmosphere

We present the results of spectral studies of thermal microwave emission from a cloudy atmosphere at 37 and 85 GHz. The experimental data are interpreted on the basis of modeling of microwave radiation transfer in the mixed-type clouds containing ice crystals of different shapes and supercooled water drops. All orders of scattering are taken into account. It is shown that polarization radiometric measurements allow one to separately determine ice-water and liquid-water contents of the clouds and also to diagnose the cloud microstructure (crystal shapes and characteristic sizes).     

Polarization of thermal microwave radiation of the cloudy atmosphere

We present the results of an experimental studies of the microwave radiation of the cloudy atmosphere indicating the presence of significant polarization contrasts in the radiation of the winter-spring clouds. The interpretation of the experimental data is made on the basis of a model of microwave radiation transfer in mixed-type clouds consisting of crystals and supercooled water drops. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 5, pp. 395–405, May., 2000.     

Light scattering by complex ice-analogue crystals

Angle-dependent light-scattering measurements on single ice analogues crystals are described. Phase functions and degree of linear polarization are measured for electrodynamically levitated crystals. A procedure for randomizing particle orientation during levitation is demonstrated. The dependence of scattering on the shape, complexity and surface roughness of the crystals is examined. The phase functions from complex crystals with smooth surfaces show little dependence on shape. There is close agreement between the measured functions and the analytic phase function for ice clouds. However, rosettes with rough surfaces have qualitatively different phase functions, with raised side and back scattering. The asymmetry parameter is typically about 0.8±0.04 and 0.63±0.05 for smooth and rough crystals, respectively. The 22° halo peak is present for smooth rosettes and aggregates but absent for rough rosettes. Two-dimensional scattering patterns from several crystals in fixed orientations are also shown. The results suggest that it may be possible to use such patterns to discriminate not only between crystals of different shape but also to obtain some information on surface properties.     

Polarization Structure of Multiply Scattered Background Components of Lidar Signals Reflected from Cloud Ice Crystals

A numerical experiment is performed to obtain the polarization characteristics of signals of a monostatic lidar intended for homogeneous cloud sensing. It is assumed that clouds consist of monodisperse randomly-oriented hexagonal ice crystals. To solve the vector radiative transfer equation, the light scattering phase matrices, preliminary calculated with the help of the beam splitting technique, are used as input data. The formation of the polarization structure of multiply scattered background signal component is studied for different scattering orders depending on crystal shapes and sizes and optical and geometrical conditions of the experiment.     

A review of the light scattering properties of cirrus

In this review paper the light scattering properties of naturally occurring ice crystals that are found in cirrus are discussed. Cirrus, also referred to as ice crystal clouds, due to their cold temperatures, consist of a variety of non-spherical ice particles which may take on a variety of geometrical forms. These geometrical forms can range from symmetric pristine hexagonal ice columns and plates, single bullets and bullet-rosettes to non-symmetric aggregates of these shapes. These aggregates may also consist of highly complex three-dimensional structures, which may themselves consist of symmetric components. Not only does cirrus consist of a wide variety of shapes but also sizes too, and these sizes can range between <10 μm to over 1 cm. With such a variety of shapes and sizes predicting the light scattering properties from such an ensemble of ice crystals is the current challenge. This challenge is important to overcome since with cirrus being so high in the Earth's atmosphere it has an important influence on the Earth-atmosphere radiation balance and consequently adds to the uncertainty of predicting climate change. This is why it is important to represent as accurately as possible the single-scattering properties of cirrus ice crystals within general circulation models so that uncertainties in climate change predictions can be reduced.In this review paper the current measurements and observations of ice crystal size and shape are discussed and how these observations relate to current ice crystal models is reviewed. The light scattering properties of the current ice crystal models are also discussed and it is shown how space-based instruments may be used to test these models. The need for particular microphysical and space-based measurements is stressed in order to further constrain ice crystal light scattering models.     

Infrared radiative transfer modelling in a 3D scattering cloudy atmosphere: Application to limb sounding measurements of cirrus

The Monte Carlo cloud scattering forward model (McClouds_FM) has been developed to simulate limb radiative transfer in the presence of cirrus clouds, for the purposes of simulating cloud contaminated measurements made by an infrared limb sounding instrument, e.g. the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). A reverse method three-dimensional Monte Carlo transfer model is combined with a line-by-line model for radiative transfer through the non-cloudy atmosphere to explicitly account for the effects of multiple scattering by the clouds. The ice cloud microphysics are characterised by a size distribution of randomly oriented ice crystals, with the single scattering properties of the distribution determined by accurate calculations accounting for non-spherical habit.A comparison of McClouds_FM simulations and real MIPAS spectra of cirrus shows good agreement. Of particular interest are several noticeable spectral features (i.e. H₂O absorption lines) in the data that are replicated in the simulations: these can only be explained by upwelling tropospheric radiation scattered into the line-of-sight by the cloud ice particles.     

Photon conservation in scattering by large ice crystals with the SASKTRAN radiative transfer model

The scattering of visible light by ice crystals and dust in radiative transfer models is challenging in part due to the large amount of scattering in the forward direction. We introduce a technique that ensures numerical conservation of photons in any radiative transfer model and that quantifies the integration error associated with highly asymmetric phase functions. When applied to a successive-orders of scatter model, the technique illustrates the high accuracy obtained in numerical integration of molecular and aerosol scattering. As well, a phase function truncation and renormalization technique is applied to scattering by ice crystals with very large size parameters, between 100 and 1000, and the scaled radiative transfer equation is solved with the spherical successive-orders model, SASKTRAN. Since computations shown this work are performed in a fully spherical model atmosphere, the computed radiances are not subject to the discontinuity at the horizon that is inherent in models using a plane–parallel assumption. The methods introduced in this work are of particular interest in modeling limb radiances in the presence of thin cirrus clouds.     

Polarization of light and its use in various problems of optics of scattering media

As light passes through scattering media, certain specific features of the polarization of radiation manifest themselves. The paper presents materials on this problem that were obtained at the Institute of Physics of the National Academy of Sciences of Belarus over recent decades. Results of experimental investigations of media that model real objects are described for the case where the dimensionless optical parameters of media and objects coincide. A method for determining the position of a diffuse light source in the atmosphere via predominant oscillations of the light vector of scattered radiation for two directions of observation is proposed. The structure of aerosol formations (smokes, dust and liquid-droplet and crystalline clouds) is interpreted based on the character of depolarization of laser radiation sounding atmosphere. The polarization of laser radiation passing through a turbid medium and reflected from it is studied. Practical applications are proposed. Fundamentals of an applied vector theory of radiation transfer, which made it possible to considerably expand notions of light scattering in strongly turbid media, are given. Studies of light propagation in encapsulated liquid crystals, which are used for solving of a large number of problems, are described. In these objects, ordinary and extraordinary rays that arise in crystals under electric voltage can give rise to a wave that is attenuated to a different degree and whose phase and polarization characteristics are varying.     

等效球模型模拟冰晶云反射的误差分析

用射线光学理论计算了具有一定尺度分布的六角冰晶粒子在可见和近红外光谱区一系列波长上(0.2～5μm)的单次散射特性.利用米氏(Mie)理论,计算了与六角冰晶具有相同截面积的球形粒子的单次散射特性.根据辐射传输理论,应用累加法,分别计算了由冰晶粒子和等效球形粒子构成的卷层云的多次散射特性,计算结果表明当入射波长λ≈3.0 μm时,等效球Mie理论可以很好地用于计算卷层云的反射特性,但是当λ＜2.8μm时,尤其在可见光区,将引起显著的误差.最后提出了计算冰云光学特性的两种方案.     

复杂大气背景下机载通信终端与无人机目标之间的激光传输特性研究

云层、气溶胶和大气分子是大气环境的主要组成部分.本文基于逐次散射法求解辐射传输方程,建立了复杂大气背景下机载无线光通信终端与地空无人机目标之间的激光传输模型.考虑真实大气背景中卷云、大气分子和气溶胶存在的情况下,数值计算了1.55 mm激光经机载通信终端发出后通过大气背景的直接传输和一阶散射传输后接收功率随无人机目标高度的变化关系,分析了飞机在云上、云中和云下以及卷云冰晶粒子有效半径、飞机与无人机之间的水平距离对接收激光信号传输功率的影响.数值结果表明:激光通过卷云传输的功率很大程度上取决于飞机在云上、云下或云中的位置;飞机与无人机目标之间的水平距离和卷云冰晶粒子的有效半径对激光直接传输和一阶散射传输影响较大;与云上大气相比,云下的大气分子和气溶胶对激光有较大的衰减.本文工作可为进一步开展地空链路上复杂大气背景对机载与低空无人机目标激光通信实验、无人机编队、指挥和组网技术的研究提供理论支撑.     

Scattering matrices for horizontally oriented ice crystals

Scattering matrices for horizontally oriented ice crystals are calculated with a code based on the geometric optics. The main physical regularities inherent to the scattering matrices are discussed. Degree of polarization of the scattered light is shown to be a qualitative criterion of number of photon trajectories that contribute effectively to the scattered light. The inverse scattering problem of retrieving aspect ratios of the horizontally oriented hexagonal ice plates from polarization of the scattered light in the bistatic sounding scheme has been proposed and discussed.     

激光在准球形边界卷云中传输的衰减和透过特性

高空卷云主要由各种不同形状的冰晶粒子组成,是地空链路上激光信号传输的重要影响因素。依据高空卷云中冰晶粒子的分布特征和散射特性,采用C版本的离散纵标法（CDISORT）,充分考虑地球球形曲率及云层冰晶粒子多次散射影响因素,研究准球形边界云层的激光透过率和衰减特性,并比较了太阳天顶角不同时平面平行模式和准球面模式下卷云大气激光透过率的差异,数值计算了三种激光波长（0.65,1.06和3.8 μm）在卷云中传输时的衰减和透过特性。计算结果表明：较小太阳天顶角（小于80°）入射时,两种模式下卷云大气激光透过率相对误差很小,其中0.65 μm激光波长入射时两种模式下的相对误差仅为1.72%,较大太阳天顶角（大于80°）入射时,两种模式下卷云大气激光透过率相对误差明显增大,0.65 μm激光波长入射时两种模式下的相对误差最大达到69%;卷云粒子单次散射时,激光在云层的衰减与卷云粒子有效半径、传输距离、光学厚度及激光波长等因素有关,随光学厚度的增加,云层的激光透过率减少,1.06 μm激光波长入射时透过率最大,3.8 μm激光波长入射时透过率最小;0.65和1.06 μm激光波长入射时,随云层粒子有效半径的增加激光透过率逐渐增加,而3.8 μm波长激光,随云层粒子有效半径的增加激光透过率逐渐减少,随相对方位角的增加,云层的激光透过率减少,且不同卷云传输模型对激光透过率也存在不同的影响。该研究工作将为开展地空链路星载、机载激光通信、激光雷达探测等工程系统中的激光信号云层传输特性的应用提供理论支持,同时也可进一步拓展为地空链路激光遥感、制导和预警等应用提供预先理论研究基础。     

Temperature dependence of ice optical constants: Implications for simulating the single-scattering properties of cold ice clouds

For a spectrum from ultraviolet to microwave and a temperature range from 160 to 270 K, the optical constants of water ice are compiled on the basis of the Kramers-Kronig relation in conjunction with existing datasets reported in literature. Significant temperature dependence is observed in both the mid-infrared and longer wavelengths. A sensitivity study at wavelengths in the infrared split window region indicates that the temperature dependence of the single-scattering properties of ice crystals is not negligible. Thus, it is necessary to take into account the temperature dependence of ice optical constants when simulating the radiative properties of cirrus clouds for various applications to remote sensing under cirrus cloud conditions.     

Estimates of radiation over clouds and dust aerosols: Optimized number of terms in phase function expansion

The bulk-scattering properties of dust aerosols and clouds are computed for the community radiative transfer model (CRTM) that is a flagship effort of the Joint Center for Satellite Data Assimilation (JCSDA). The delta-fit method is employed to truncate the forward peaks of the scattering phase functions and to compute the Legendre expansion coefficients for re-constructing the truncated phase function. Use of more terms in the expansion gives more accurate re-construction of the phase function, but the issue remains as to how many terms are necessary for different applications. To explore this issue further, the bidirectional reflectances associated with dust aerosols, water clouds, and ice clouds are simulated with various numbers of Legendre expansion terms. To have relative numerical errors smaller than 5%, the present analyses indicate that, in the visible spectrum, 16 Legendre polynomials should be used for dust aerosols, while 32 Legendre expansion terms should be used for both water and ice clouds. In the infrared spectrum, the brightness temperatures at the top of the atmosphere are computed by using the scattering properties of dust aerosols, water clouds and ice clouds. Although small differences of brightness temperatures compared with the counterparts computed with 4, 8, 128 expansion terms are observed at large viewing angles for each layer, it is shown that 4 terms of Legendre polynomials are sufficient in the radiative transfer computation at infrared wavelengths for practical applications.     

Submillimeter-wave scattering indicatrices for oversized water drops and ice particles in a cloud

Angular distribution functions for the intensity scattered by oversized water drops and ice particles in the raining clouds are considered in the paper for five wavelength values in the submillimeter (SubMM) wave band. It is shown that the obtained scattering indicatrices may be used to explain the effect of abnormal SubMM backscatter originating in the water-ice phase transition of oversized drops, as well as the effect of small-angle scattering on abnormal backscatter emission.     

相态对冰水混合云微观特性反演的影响研究

当云层的温度在－40℃~0℃之间时, 云层中会存在冰和水两种相态的云滴, 其散射特性与纯水云以及纯冰云特性有较大差异, 因此遥感反演混合相云层的微观和宏观物理特性具有重要的意义。本文采用冰水双层球模型模拟了冰水混合云中的云滴, 利用Mie理论计算了纯水、纯冰和冰水颗粒的单次散射特性, 分析了单次散射相函数, 不对称因子, 单次散射反照率等随着有效半径、相态、内外半径比等的变化特性。利用离散纵标法(DISORT)计算了水云和冰云对0.75 μm、2.16 μm和3.3 μm的双向反射函数, 讨论了利用纯水滴和纯冰滴反演冰水混合云滴的误差。分析结果表明, 利用0.75 μm和2.16 μm的太阳光反演冰水混合云的光学厚度和有效半径时, 光学厚度误差较大, 有效半径误差较小; 结合0.75 μm和3.3 μm的太阳光反演冰水混合云的光学厚度和有效半径时, 光学厚度误差较小, 有效半径误差较大, 其会高估其有效半径; 另外结合0.75 μm和3.3 μm这两个波长的反射函数反演冰水云的冰水混合比更为有效。     

可见光波段卷云散射特性的研究

卷云一般分布在对流层上部到平流层下部,主要由各种形状的冰晶粒子组成。卷云散射特性的研究利用了前期建立的单个冰晶粒子散射性质数据库,并且假定卷云中粒子谱分布符合Г分布,计算得出可见光波段卷云平均散射性质：消光效率因子、吸收效率因子、单次散射反照率、非对称因子与有效尺度以及波长的变化关系。其次对计算得出的变化曲线进行分析得...     

A strong ice cloud event as seen by a microwave satellite sensor: Simulations and observations

In this article, brightness temperatures observed by channels of the Advanced Microwave Sounding Unit-B (AMSU-B) instrument are compared to those simulated by a radiative transfer model, which can take into account the multiple scattering due to ice particles by using a discrete ordinate iterative solution method. The input fields, namely, the pressure, temperature, humidity, and cloud water content are taken from the short range forecast from the Met Office mesoscale model (UKMES). The comparison was made for a case study on the 25 January 2002 when a frontal system associated with significant cloud was present over the UK. It is demonstrated that liquid clouds have maximum impact on channel 16 of AMSU whereas ice clouds have maximum impact on channel 20. The main uncertainty for simulating microwave radiances is the assumptions about microphysical properties, such as size distribution, shape and orientation of the cloud particles, which are not known in the mesoscale model. The article examines the impact of these parameters on the cloud signal. The polarisation signal due to oriented ice particles at these frequencies is also discussed.     

太赫兹波被动遥感卷云微物理参数的敏感性试验分析

太赫兹波长和典型卷云的冰晶粒子尺度处于同一量级,其在遥感卷云微物理参数(粒子尺度和冰水路径)方面具有广阔的应用前景.为了评估卷云微物理参数对太赫兹波传输特性的影响及其在太赫兹波段的敏感性,基于大气辐射传输模式分别模拟计算了晴空和有云条件下大气层顶的太赫兹辐射光谱特征,分析了这两种条件下辐射亮温差值的特点,研究了卷云冰晶粒子形状、粒子尺度及冰水路径对太赫兹辐射传输特性的影响,并定量计算了相关敏感系数.结果表明:卷云冰晶粒子形状、粒子尺度、冰水路径等对太赫兹波传输特性均有不同程度的影响,卷云效应也因通道频率而异,太赫兹波对卷云的粒子尺度和冰水路径有较高的敏感性,是理论上被动遥感卷云微物理特性的最佳波段.研究结果对于进一步发展太赫兹波被动遥感卷云技术、提高卷云参数的反演精度具有重要意义.