Modeling single-scattering properties of small cirrus particles by use of a size-shape distribution of ice spheroids and cylinders

In this study, we model single-scattering properties of small cirrus crystals using mixtures of polydisperse, randomly oriented spheroids and cylinders with varying aspect ratios and with a refractive index representative of water ice at a wavelength of 1.88 μm. The Stokes scattering matrix elements averaged over wide shape distributions of spheroids and cylinders are compared with those computed for polydisperse surface-equivalent spheres. The shape-averaged phase function for a mixture of oblate and prolate spheroids is smooth, featureless, and nearly flat at side-scattering angles and closely resembles those typically measured for cirrus. Compared with the ensemble-averaged phase function for spheroids, that for a shape distribution of cylinders shows a relatively deeper minimum at side-scattering angles. This may indicate that light scattering from realistic cirrus crystals can be better represented by a shape mixture of ice spheroids. Interestingly, the single-scattering properties of shape-averaged oblate and prolate cylinders are very similar to those of compact cylinders with a diameter-to-length ratio of unity. The differences in the optical cross sections, single-scattering albedo, and asymmetry parameter between the spherical and the nonspherical particles studied appear to be relatively small. This may suggest that for a given optical thickness, the influence of particle shape on the radiative forcing caused by a cloud composed of small ice crystals can be negligible.     

Dust aerosol forward scattering effects on ground-based aerosol optical depth retrievals

Monte Carlo radiative transfer calculations are performed to examine the forward scattering effects on retrievals of dust aerosol optical depth (AOD) from ground-based instruments. We consider dust aerosols with different AOD, effective radius and imaginary refractive index at 0.5 μm wavelength. The shape of dust aerosols is assumed to be spheroids and the equivalent spheres that preserve both volume and projected area (V/P) are also considered. The single-scattering albedos and asymmetry factors of spheroids and V/P-equivalent spheres have small differences, but the scattering phase functions are very different for the scattering angle range ∼90-180°. The relative errors of retrieved AOD caused by forward scattering effects due to the differences between the single-scattering properties of spheroids and spheres are similar. It is shown that at solar zenith angle (SZA) smaller than ∼70° the effect of the forward scattering is generally small although the relative errors in retrieved AOD can be as large as −10% when r_e=2. However, the largest relative errors, which can reach −40%, appear at high SZA (>∼70°) with AOD larger than 1. This is not caused by the increase of forward scattering intensity, but is due to the strong attenuation of solar direct beam.     

Efficient Nd:YAG pumped mid-IR laser based on cascaded KTP and ZGP optical parametric oscillators and a ZGP parametric amplifier

Efficient conversion into the mid-IR of a low pulse-energy (2.5 mJ) Nd:YAG laser is achieved by cascaded KTiOPO₄ (KTP) and ZnGeP₂ (ZGP) optical parametric oscillators followed by a ZGP optical parametric amplifier. The first stage 2.13 μm degenerate KTP OPO uses four KTP crystals in a walk-off compensated geometry and an elliptical pump beam focal geometry to produce up to 2.2 W from 6.3 W incident. The 2.13 μm e-ray pumps a Type-I ZGP OPO, which produces 0.5 W of light in the 3.8-4.8 μm spectral region that in turn is amplified by a 2.13 μm o-ray pumped optical parametric amplifier generating 0.84 W with an M² of <2.     

表面粗糙度对黑云斜长片麻岩不同探测角偏振度波谱影响研究

以黑云斜长片麻岩为例,探讨了表面粗糙度因子在350～2 500nm波段对岩石不同天顶角及方位角偏振度探测的影响,并在特征波段进行了定量分析。表明不同粗糙度的岩石表面,入射天顶角一定时,在镜面反射方位,偏振度随探测天顶角先是上升,到达最高值后又开始下降。(520±10)nm波段的偏振度在探测天顶角方向达到的最大峰值与表面粗糙度进行相关分析,发现他们呈幂函数关系。回归方程为:y=0.604x-0.297,可决系数R2为0.985 4。峰值所在天顶角度与表面粗糙度相关模型为,y=3.419 4x+51.584,y<90°,模型可决系数R2为0.817 7。偏振度随探测方位角变化的规律为以180°方位时偏振度最大,向两侧随探测方位离180°越远逐渐越低,并趋于0。(520±10)nm波段的偏振度在探测方位角180°方向达到的最大峰值与表面粗糙度进行相关分析,发现他们呈幂函数关系。回归方程为:y=0.582 2x-0.333,可决系数R2为0.984 3。对以上回归模型进行F检验,发现偏振度峰值及所对应角度与粗糙度之间存在显著的相关关系。     

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.     

Dependence of ice crystal optical properties on particle aspect ratio

The single-scattering properties of randomly oriented hexagonal ice crystals have been extensively used to study the bulk radiative properties of ice clouds. It has been reported in the literature that the asymmetry factors of these particles vary with their aspect ratios in a “V-pattern” with the minimum at a unit aspect ratio. However, this phenomenon was not explained in the previous studies. The present paper reports on an in-depth analysis of the optical properties of hexagonal ice crystals. It is shown that the delta-transmission is primarily responsible for the aforementioned “V-pattern” variation of the asymmetry factor as a function of the aspect ratio. Additionally, the term (1?f_δ)g is also partially responsible for the large values of the total asymmetry factor in the case of small aspect ratios, where f_δ indicates the ratio of the amount of the energy associated with the delta-transmission rays to the total amount of the scattered energy and g is the asymmetry factor associated with the geometric optics rays.     

Evaluation of errors from neglecting polarization in the forward modeling of O2A band measurements from space, with relevance to CO2 column retrieval from polarization-sensitive instruments

Sensitivity studies have been performed to evaluate the errors resulting from ignoring polarization in analyzing spectroscopic measurements of the O₂A band from space, using the Orbiting Carbon Observatory (OCO) as a test case. An 11-layer atmosphere, with both gas and aerosol loading, and bounded from below by a lambertian reflecting surface, was used for the study. The numerical computations were performed with a plane-parallel vectorized discrete ordinate radiative transfer code. Beam and viewing geometry, surface reflectance and aerosol loading were varied one at a time to evaluate and understand the individual errors. Different behavior was observed in the line cores and the continuum because of the different paths taken by the photons in the two cases. The errors were largest when the solar zenith angle was high, and the aerosol loading and surface reflectance low. To understand the effect of neglecting polarization on CO₂ column retrievals, a linear error analysis study was performed on simulated measurements from the OCO spectral regions, viz. the 1.61 and 2.06 μm CO₂ bands and the O₂A band. It was seen that neglecting polarization could introduce errors as high as 10 ppm, which is substantially larger than the required retrieval precision of ∼2 ppm. A variety of approaches, including orders of scattering, spectral binning and the use of lookup tables are being explored to reduce the errors.     

黑松冠层反射光谱方向特征分析

松萎蔫病是松属树种的一种毁灭性病害，小范围甚至单木水平的森林病虫害的早期诊断对森林资源保护与可持续发展尤为重要。以感染松萎蔫病的黑松为研究对象，通过采集不同感病时期的黑松冠层的多角度光谱数据，分析不同特征波段的方向反射特征，总结不同感病程度黑松的冠层特征波段反射率的变化规律。结果显示：(1）在俯视观测时，在主平面方向的后向散射方向的反射率大于前向散射方向的反射率，并且在后向散射方向，四个波段的四个感病时期约在40°的观测天顶角出现热点效应；无论在主平面还是主垂面，蓝光波段（450 nm）与近红外波段（810 nm）的黑松冠层0°天顶角反射率呈现出感病初期＞健康＞感病中期＞感病末期的变化规律，红光波段（680 nm）和绿光波段（560 nm）的黑松冠层0°天顶角反射率呈现出健康≈感病初期＞感病中期≈感病末期的变化规律。在所有方位角，冠层反射率随着观测天顶角的增加而增大。(2）在仰视观测时，在主平面方向的后向散射方向的反射率小于前向散射方向的反射率，并且在方位角为0°时，4个波段反射率都是较大的；无论在主平面还是主垂面，蓝光波段（450 nm）绿光波段（560 nm）和红光波段（680 nm）的冠层反射率的大小呈现出感病初期＞健康>感病末期>感病中期的变化规律，近红外波段（810 nm）冠层反射率的大小呈现出感病初期＞健康>感病中期>感病末期的变化规律；在所有方位角，冠层反射率随着观测仰角的增加而减小。(3）黑松冠层反射光谱在俯视和仰视观测时，各个特征波段的二向性反射率的各向异性最强的是主平面，最弱的是主垂面，且主垂面的前向和后向反射率会呈现对称性，即“镜面反射”；各个特征波段在感病末期，黑松冠层反射率随观测天顶角的变化幅度较大，其他几个时期反射率随观测天顶角的变化幅度不明显。研究结果显示的树冠的不同角度的波段反射方向性特征为以后不同尺度的无人机监测的准确性与可靠性奠定基础，也为发展近地面便携式森林病虫害实时监测系统打下了基础。     

黑碳团簇气溶胶混合生长的红外吸收特性及长波辐射效应

黑碳气溶胶是当前气溶胶辐射强迫评估中最不确定的因子.本文通过构建黑碳的微物理模型,分别模拟了新鲜状态的黑碳气溶胶和混合生长(老化)后被硫酸盐包裹的黑碳气溶胶,利用叠加T矩阵方法计算获得了具有团簇形态和多成分混合的黑碳气溶胶红外吸收特性,通过大气辐射传输模型模拟了黑碳气溶胶的长波辐射强迫,分析了典型理化参数的敏感性.发现黑碳混合生长可以显著增强其大气层顶的长波辐射强迫,最高可达3倍.而且,包裹黑碳的硫酸盐半径越大,将明显增强大气层顶的黑碳长波辐射强迫.这些发现将有助于降低黑碳气溶胶气候效应评估的不确定性.     

观测几何对大气偏振光谱的影响

研究大气偏振光谱在太阳天顶角SZA、观测天顶角VZA和相对方位角RAA构成的观测几何变化时的特征。采用离散坐标法求解大气矢量辐射传输方程,结合真实大气环境与结构参数,以MODTRAN太阳参照光谱仿真大气层顶初始光谱,考虑下垫面反照、气溶胶垂直分层与成分混合比及痕量气体吸收的影响,仿真计算相对方位角为0°和90°时0.4~3 μm波段大气偏振光谱,丰富大气偏振光谱信息。仿真结果表明：(1)相对方位角为0°时,偏振度DoP和Q/I随SZA的变化分别呈“V形”和倒“U”形, U/I很微弱,量级低于10-7,V/I在任何观测几何下都很微弱;(2)晨暮时段,若太阳不在视场内,大气偏振度在波长0.51和2.75 μm附近为极大值,在1.5 μm附近为极小值。因此,在不同的观测几何条件下,选择相应的波段和偏振信号,将有利于实体目标探测。     

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50-230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (<30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain ∼1-3 cm⁻¹ above the laser thresholds (>1 kW cm⁻²) and provide 10 ps-1 μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6-5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A₁) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm GW⁻¹ and the optical threshold intensity is ∼100 kW cm⁻².     

VLIDORT: A linearized pseudo-spherical vector discrete ordinate radiative transfer code for forward model and retrieval studies in multilayer multiple scattering media

We describe a new vector discrete ordinate radiative transfer model with a full linearization facility. The VLIDORT model is designed to generate simultaneous output of Stokes vector light fields and their derivatives with respect to any atmospheric or surface property. We develop new implementations for the linearization of the vector radiative transfer solutions, and go on to show that the complete vector discrete ordinate solution is analytically differentiable for a stratified multilayer multiply scattering atmospheric medium. VLIDORT will generate all output at arbitrary viewing geometry and optical depth. The model has the ability to deal with attenuation of solar and line-of-sight paths in a curved atmosphere, and includes an exact treatment of the single scatter computation. VLIDORT also contains a linearized treatment for non-Lambertian surfaces. A number of performance enhancements have been implemented, including a facility for multiple solar zenith angle output. The model has been benchmarked against established results in the literature.     

雪的热辐射多角度偏振特性研究

针对现今遥感对地面目标自身热辐射偏振特性研究的需要,对雪的热辐射多角度偏振特性进行测量,分析探测天顶角、探测方位角、波段、偏振角四个因子对雪的热辐射特性的影响。结果表明：雪的辐射亮度和亮度温度均随探测天顶角的增大而增大,当探测天顶角＞30°时,增长速度加快,且探测天顶角对雪的亮度温度的影响较辐射亮度更为显著;探测方位角的变化对雪的辐射亮度和亮度温度有一定的影响,且对雪的亮度温度的影响较辐射亮度更为显著;波段的变化对雪的辐射亮度和亮度温度均有显著影响,相比而言,其对辐射亮度的影响更为显著;偏振角的变化对雪的辐射亮度和亮度温度有一定的影响,且对亮度温度的影响较辐射亮度较为显著。该研究成果为应用遥感技术开展雪的热红外定量研究提供了新的思路与方法,具有重要的理论意义和广阔的应用前景。     

Microwave scattering properties of sand particles: Application to the simulation of microwave radiances over sandstorms

The single-scattering properties of sand/dust particles assumed to be ellipsoids are computed from the discrete dipole approximation (DDA) method at microwave frequencies 6.9-89.0 GHz in comparison with the corresponding Lorenz-Mie solutions. It is found that the single-scattering properties of sand particles are strongly sensitive to the shapes of the particles. The bulk scattering properties of sandstorms composed of spherical or nonspherical particles are investigated by averaging the single-scattering properties of these particles over log-normal particle size distributions. Furthermore, a vector radiative transfer model is used to simulate microwave radiances. The microwave brightness temperatures in the vertical polarization model are essentially not sensitive to sand particle habit, whereas microwave brightness temperature polarization differences are influenced by particle habit. It is shown that microwave brightness temperatures and brightness temperature polarization differences may be useful for estimating the effective particle sizes and mass loading of sandstorms.     

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。     

Numerical accuracy of “equivalent” spherical approximations for computing ensemble-averaged scattering properties of fractal soot aggregates

Numerical accuracy is quantitatively assessed in conjunction with the application of four “equivalent” spherical approximations in the computation of the optical properties of small aggregate soot particles. The approximations are based on equal volume, equal surface area, the radius of gyration, and a collection of independent spheres with the same volume and the same volume-to-projected area ratio as the original nonspherical particle. A diffusion-limited cluster-cluster aggregation algorithm is used to specify the geometries of soot particles. Furthermore, the Generalized Multi-particle Mie (GMM) method is utilized to compute the single-scattering properties of individual soot aggregate particles assumed to be randomly oriented in space. The ensemble-averaged single-scattering properties of the particles are obtained by accounting for the probability distribution functions (PDF) of the number of monomers per aggregate at two wavelengths, 0.628 and 1.1 μm. It is shown that all of the aforementioned equivalent-spherical approximations lead to large errors in the computation of the phase function.     

Microwave radiative transfer in scattering atmosphere: Effects of complex permittivity of ice spherical crystals

This paper presents the effects of ice particle's complex permittivity uncertainties on the scattering properties and upwelling brightness temperatures of cloudy atmospheres at the Advanced Microwave Sounding Unit-B (AMSU-B) channel frequencies of 89, 150 and 183 GHz. We investigated the mean deviations of ice particle's optical efficiencies and asymmetry parameters due to the uncertainties in the real and imaginary parts of its complex permittivities. We assumed that the true values of ice particle's permittivity are, respectively, within ±20% for the imaginary part and ±5% for the real part of the permittivity values given by the model of Hufford. Microwave radiative transfer calculations were performed to estimate the absolute errors of brightness temperatures due to uncertainties in ice particle's permittivities. Ice particles were taken to be spherical and their diameters were chosen in the range of 40-4000 μm. Gamma-size distribution was employed in computing volume scattering properties and the effective diameters were 70, 100 and 150 μm with an effective variance being 0.25. We found that ±20% uncertainty in the imaginary part of ice particle's permittivity resulted in only about 10% mean deviations in the absorption efficiencies at the three AMSU-B channel frequencies. However, an uncertainty of ±5% in the real part resulted in more than 15% mean deviations in both scattering and extinction efficiencies, especially significant at the frequency of 183 GHz. The absolute variations of the emerging brightness temperature from the cloudy atmosphere due to uncertainties in the permittivity were found to be more than 1 K, which is already significant compared with the sensitivities achieved with today's technology for millimeter wave radiometers.     

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I-V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm² at 2.5 V/μm.     

Extended discrete-ordinate method considering full polarization state

This paper presents an extension to the standard discrete-ordinate method (DOM) to consider generalized sources including: beam sources which can be placed at any (vertical) position and illuminate in any direction, thermal emission from the atmosphere and angularly distributed sources which illuminate from a surface as continuous functions of zenith and azimuth angles. As special cases, the thermal emission from the surface and deep space can be implemented as angularly distributed sources. Analytical-particular solutions for all source types are derived using the infinite medium Green's function. Radiation field zenith angle interpolation using source function integration is developed for all source types. The development considers the full state of polarization, including the sources (as applicable) and the (BRDF) surface, but the development can be reduced easily to scalar problems and is ready to be implemented in a single set of code for both scalar and vector radiative transfer computation.     

A fast linearized pseudo-spherical two orders of scattering model to account for polarization in vertically inhomogeneous scattering-absorbing media

We calculate the reflection matrix for the first two orders of scattering in a vertically inhomogeneous, scattering-absorbing medium. We take full account of polarization and perform a complete linearization (analytic differentiation) of the reflection matrix with respect to both the inherent optical properties of the medium and the surface reflection condition. Further, we compute a scalar-vector correction to the total intensity due to the effect of polarization; this correction is also fully linearized. The solar beam attenuation has been computed for a pseudo-spherical atmosphere.Results from the two orders of scattering (2OS) model have been tested against scalar intensities for an inhomogeneous atmosphere, and against Stokes vector results for a homogeneous atmosphere. We have also performed backscatter simulations of reflected sunlight in the O₂A band for a variety of geometries, and compared our results with those from a full vector multiple scattering code. Our results are exact in the center of strong lines and most inaccurate in the continuum, where polarization is least significant. The s- and p-polarized radiances are always computed very accurately. The effect of gas absorption optical depth, solar zenith angle, viewing geometry, surface albedo and wind speed (in the case of ocean glint) on the intensity, polarization and corresponding weighting functions have been investigated. It is shown that the 2OS model provides fast and reliably accurate polarization corrections to the scalar-model radiance and weighting function fields. The model can be implemented in operational retrieval algorithms as an adjunct radiative transfer code to deal with polarization effects.