On the retrieval of ice cloud particle shapes from POLDER measurements

Shapes of ice crystals can significantly affect the radiative transfer in ice clouds. The angular distribution of the polarized reflectance over ice clouds strongly depends on ice crystal shapes. Although the angular-distribution features of the total or polarized reflectance over ice clouds imply a possibility of retrieving ice cloud particle shapes by use of remote sensing data, the accuracy of the retrieval must be evaluated. In this study, a technique that applies single ice crystal habit and multidirectional polarized radiance to retrieve ice cloud particle shapes is assessed. Our sensitivity studies show that the retrieved particle shapes from this algorithm can be considered good approximations to those in actual clouds in calculation of the phase matrix elements. However, this algorithm can only work well under the following conditions: (1) the retrievable must be overcast and thick ice cloud pixels, (2) the particles in the cloud must be randomly oriented, (3) the particle shapes and size distributions used in the lookup tables must be representative, and (4) the multi-angle polarized measurements must be accurate and sufficient to identify ice cloud pixels of randomly oriented particles. In practice, these conditions will exclude most of the measured cloud pixels. Additionally, because the polarized measurements are only sensitive to the upper cloud part not deeper than an optical thickness of 4, the retrieved particle shapes with the polarized radiance may only approximate those in the upper parts of the clouds. In other words, for thicker clouds with vertical inhomogeneity in particle shapes, these retrieved particle shapes cannot represent those of whole clouds. More robust algorithm is needed in accurate retrieval of ice cloud particle shapes.     

Photo of a synthetic atom [submillimeter microwave radiationproduction]

Electrons are trapped around thicker sections of selectively etched wire which act as isolated positive nuclei. The trapped electrons are made visible by the glowing clouds around the thicker sections of wire. The object of this work is to improve the performance of an orbitron microwave maser in the production of submillimeter microwave radiation     

卷云短波红外辐射特性

 采用离散纵标法耦合大气分子吸收,模拟计算了卷云大气的反射特性。研究了在短波波段卷云辐射性质随波长、卷云光学厚度、卷云有效尺度、云高和卷云中冰晶粒子形状等的变化关系,分析了卷云对大气红外背景辐射的影响。结果表明：在2.7 μm的水汽强吸收带上,卷云的出现明显增强了该波段的大气背景辐射,反射率随光学厚度和云高增大而增大。     

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.     

Rise and flow of volcanic clouds observed from the ground and from satellites

Typical patterns of volcanic clouds from Mt. Sakurajima are discussed with photographs and satellite images. Volcanic clouds are classified into three types, i.e., eruption cloud, steady flow as a plume, and sequential puffs, according to the variation of the ejection activity with time. While the rise of eruption clouds depends on the strength of the eruption, the vertical shapes of the plumes are sensitive to the velocity of the cross winds around and above the summit of the volcanic mountain. Very strong winds cause blowing down and bouncing up of the plume along the mountain lee wave. Various patterns of horizontal dispersion are observed from the ground and from satellites; typical ones are linear advection, fan- and belt-type spreads, and flat stagnation. These patterns are essentially determined by the wind shear within the vertical thickness of the volcanic cloud.     

波破碎下海洋表面微波辐射率的一种模式

利用有效介质近似理论方法，本文提出了计算强风驱动下海浪破碎的海洋表面微波辐射率的一种修正模式。该模式修正Pandey等人的经验模式，通过把海浪破碎的白冠层等效为海水和空气的复合介质，并利用复合介质有效介电常数的计算方法，在该模式中充分考虑了海洋表面水滴形态、白冠层厚度和覆盖率对海洋表面微波辐射率的影响。实验表明，该模式在不同风速、不同水滴形态以及微波频率下（6.6GHz,10.7GHz,37GHz）计算的海洋表面微波辐射率与实测结果一致。     

Polarization Effects Due to Scattering of Microwave Radiation in ihe Summer Cloudy Atmosphere

We present the results of the spectral analysis of polarization of the thermal microwave emission from the summer cloudy atmosphere at 37 and 94 GHz. A significant polarization of the microwave emission from summer clouds is revealed, which is attributed to scattering of the thermal emission from the atmosphere by melted ice crystals in the melting layer. Based on a model of polarized radiation transfer in the mixed clouds containing melted ice crystals and supercooled water drops, we interpret the experimental data with allowance for all orders of scattering. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 4, pp. 314–326, April 2005.     

Sensitivity of depolarized lidar signals to cloud and aerosol particle properties

Measurements from depolarized lidars provide a promising method to retrieve both cloud and aerosol properties and a versatile complement to passive satellite-based sensors. For lidar observations of clouds and aerosols, multiple scattering plays an important role in the scattering process. Monte Carlo simulations are carried out to investigate the sensitivity of lidar backscattering depolarization to cloud and aerosol properties. Lidar parameters are chosen to be similar to those of the upcoming space-based CALIPSO lidar. Cases are considered that consist of a single cloud or aerosol layer, as well as a case in which cirrus clouds overlay different types of aerosols. It is demonstrated that besides thermodynamic cloud phase, the depolarized lidar signal may provide additional information on ice or aerosol particle shapes. However, our results show little sensitivity to ice or aerosol particle sizes. Additionally, for the case of multiple but overlapping layers involving both clouds and aerosols, the depolarized lidar contains information that can help identify the particle properties of each layer.     

Microwave fourier transform spectroscopy: Linewidth effects in the low pressure limit

We describe theoretically the line shapes obtained with pulsed microwave Fourier Transform Spectroscopy carried out on a static gas at very low pressures in a waveguide absorption cell. A density-matrix formalism is used to study the interaction of a two-level quantum system with a classical, pulsed, travelling wave electric field in a rectangular waveguide. Neglecting intermolecular collisions, the effects of the molecular velocity distribution the linewidths of microwave transitions are considered, allowing for Doppler broadening, wall collisions and microwave-field inhomogeneities. General expressions for the decay function of sample polarisation are derived and examples for the resulting line shapes are given.     

Microwave backscattering from the sea surface with breaking waves

Based on the effective medium approximation theory of composites, the whitecap-covered sea surface is treated as a medium layer of dense seawater droplets and air. Two electromagnetic scattering models of randomly rough surface are applied to the investigation of microwave backscattering of breaking waves driven by strong wind. The shapes of seawater droplets are considered by calculating the effective dielectric constant of the whitecap layer. The responses of seawater droplets shapes, such as sphere and ellipsoid, to the backscattering coefficient are discussed. Numerical results of the models are in good agreement with the experimental measurements of horizontally and vertically polarized backscattering at microwave frequency 13.9GHz and different incidence angles.     

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...     

An adaptive approach for primitive shape extraction from point clouds

In the paper, an adaptive approach for primitive shape extraction from point clouds is presented. The approach extends RANSAC segmentation algorithm in two ways: adaptive primitive shape detection based on histogram analysis of points’ deviations from their corresponding fitted primitive shapes; getting boundary points of the fitted primitive shapes and trimming uniform points of primitive shapes according to boundaries. The data structure of a fitted primitive shape contains two parts: parameter vectors that define the primitive shape; boundary points of the primitive shape. By placing uniform points on primitive shapes and trimming away the points outside of boundaries, we got trimmed uniform points each fitted primitive shapes. Afterwards, we got mesh models of primitive shapes by exerting Delaunay algorithm on each set of trimmed uniform points. At last, we output the final model by calibrating primitive shapes’ positions and orientations and trimming primitive shapes according to their intersection lines.     

A simple new radiative transfer model for simulating the effect of cirrus clouds in the microwave spectral region

The upwelling atmospheric radiation in the millimeter wave spectral range is influenced by the presence of cirrus clouds. A plane parallel radiative transfer model which can take into account the effect of multiple scattering by ice particles in the cirrus has been developed and is used to simulate the brightness temperatures as they would be measured by a satellite instrument. The model uses an iterative procedure to solve the radiative transfer equation. The formulation of the model is such that it can easily be adapted to treat the full specific intensity vector instead of just the scalar total intensity. A convergence test for the model is explained and two cirrus cloud scenarios are simulated. The results illustrate the linearity of microwave radiative transfer for not too strong cirrus clouds in this frequency region.     

Microwave radiative transfer intercomparison study for 3-D dichroic media

Three different numerical methods capable of solving the radiative transfer of microwave radiation within 3-D dichroic media are compared. A case study, represented by an intense rain shaft populated by perfectly oriented oblate raindrops, is analysed in detail, including a discussion of the behaviour of all four Stokes components.Results demonstrate an acceptable agreement between all Monte Carlo methods. The method based on a discrete ordinates scheme agrees only qualitatively with the Monte Carlo outputs. Because of its lower computational cost the backward Monte Carlo technique based on importance sampling represents the most efficient way to face passive microwave radiative transfer problems related to optically thick 3-D structured clouds including non-spherical preferentially oriented hydrometeors.     

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.     

Increase in the average radiation power of a plasma relativistic microwave generator

The operation of a pulse-periodic plasma relativistic microwave generator with a pulsed power of 50 MW was experimentally studied. A change in the shapes of the electron collector and output unit allowed a significant increase in the average emission power. The microwave pulse duration was increased from 30 to 70 ns, and the repetition rate of microwave pulses was increased from 5 to 50 Hz.     

Review of collective beam instabilities in electron-positron storage rings

The review of studies of collective beam instabilities in electron-positron storage rings is presented. The processes of excitation and suppression of the longitudinal microwave instability, transverse mode coupling instability, longitudinal and transverse multi-bunch instabilities, and instabilities induced by an interaction of a beam with ions or electron clouds are discussed. Important equations for estimation of the threshold beam currents and the rise time of instabilities, as well as the references to the major original works are given. The methods for diagnostics and suppression of instabilities are considered using specific examples.     

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.     

Atom number calibration in absorption imaging at very small atom numbers

Cold atom experiments often use images of the atom clouds as their exclusive source of experimental information. The most commonly used technique is absorption imaging, which provides accurate information about the shapes of the atom clouds, but requires care when seeking the absolute atom number for small atom samples. In this paper, we present an independent, absolute calibration of the atom numbers. We directly compare the atom number detected using dark-ground imaging to the one observed by fluorescence imaging of the same atoms in a magneto-optical trap. We normalise the signal using single-atom resolved fluorescence imaging. In order to be able to image the absorption of the very low atom numbers involved, we use diffractive dark-ground imaging as a novel, ultra-sensitive method of in situ imaging for untrapped atom clouds down to only 100 atoms. We demonstrate that the Doppler shift due to the acceleration of the atoms by the probe beam has to be taken into account when measuring the atom-number.     

All-weather long-wavelength infrared free space optical communications

Long-wavelength infrared radiation possesses superior transmission through common atmospheric problems such as fog, clouds, and smoke than the shorter wavelength laser sources in use today. Recent improvements in LWIR laser and modulator design makes possible reliable optical replacements for radio and microwave communications links in many applications. This paper describes components and techniques developed for high-speed, full-duplex all-weather infrared communications systems.