全极化微波辐射计姿态对观测亮温的影响及消除

针对全极化微波辐射计精确探测的需求, 研究了辐射计姿态对于观测亮温的影响以及亮温误差校正. 建立了姿态偏移与观测入射角以及极化旋转角的关系, 模拟了观测亮温随观测入射角以及极化旋转角波动的变化; 仿真了姿态偏移情况下的辐射计原始观测亮温;运用一种基于辐射传输模型的姿态补偿方法, 以垂直极化亮温和第三Stokes参数亮温为例, 对原始观测亮温展开误差校正. 研究显示, 该方法能够有效去除辐射计姿态偏移对观测亮温造成的影响, 校正结果满足辐射计数据预处理的误差精度要求. 关键词： 全极化微波辐射计 Stokes参数 观测入射角 极化旋转角     

A single-scattering approximation for infrared radiative transfer in limb geometry in the Martian atmosphere

We present a single-scattering approximation for infrared radiative transfer in limb geometry in the Martian atmosphere. It is based on the assumption that the upwelling internal radiation field is dominated by a surface with a uniform brightness temperature. It allows the calculation of the scattering source function for individual aerosol types, mixtures of aerosol types, and mixtures of gas and aerosol. The approximation can be applied in a Curtis-Godson radiative transfer code and is used for operational retrievals from Mars Climate Sounder measurements. Radiance comparisons with a multiple scattering model show good agreement in the mid- and far-infrared although the approximate model tends to underestimate the radiances in realistic conditions of the Martian atmosphere. Relative radiance differences are found to be about 2% in the lowermost atmosphere, increasing to ∼10% in the middle atmosphere of Mars. The increasing differences with altitude are mostly due to the increasing contribution to limb radiance of scattering relative to emission at the colder, higher atmospheric levels. This effect becomes smaller toward longer wavelengths at typical Martian temperatures. The relative radiance differences are expected to produce systematic errors of similar magnitude in retrieved opacity profiles.     

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 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.     

Validation of the community radiative transfer model

To validate the Community Radiative Transfer Model (CRTM) developed by the U.S. Joint Center for Satellite Data Assimilation (JCSDA), the discrete ordinate radiative transfer (DISORT) model and the line-by-line radiative transfer model (LBLRTM) are combined in order to provide a reference benchmark. Compared with the benchmark, the CRTM appears quite accurate for both clear sky and ice cloud radiance simulations with RMS errors below 0.2 K, except for clouds with small ice particles. In a computer CPU run time comparison, the CRTM is faster than DISORT by approximately two orders of magnitude. Using the operational MODIS cloud products and the European Center for Medium-range Weather Forecasting (ECMWF) atmospheric profiles as an input, the CRTM is employed to simulate the Atmospheric Infrared Sounder (AIRS) radiances. The CRTM simulations are shown to be in reasonably close agreement with the AIRS measurements (the discrepancies are within 2 K in terms of brightness temperature difference). Furthermore, the impact of uncertainties in the input cloud properties and atmospheric profiles on the CRTM simulations has been assessed. The CRTM-based brightness temperatures (BTs) at the top of the atmosphere (TOA), for both thin (τ<5) and thick (τ>30) clouds, are highly sensitive to uncertainties in atmospheric temperature and cloud top pressure. However, for an optically thick cloud, the CRTM-based BTs are not sensitive to the uncertainties of cloud optical thickness, effective particle size, and atmospheric humidity profiles. On the contrary, the uncertainties of the CRTM-based TOA BTs resulting from effective particle size and optical thickness are not negligible in an optically thin cloud.     

Transient heat transfer in a spherical protective material, submitted to flux and mixed boundary conditions: an investigation on zirconia

An analytic solution is presented for describing combined radiation and conduction heat transfer in a spherical fiber thermal protection exposed to combined radiative and convective heating. The solution includes the equation of radiative transfer within the material, coupled to a transient energy equation that contains both radiative and convective terms. At elevated temperatures radiative transfer becomes important, and if several hot surfaces view each other, the radiation exchange process must be considered carefully. Some thermal protections are partially transparent to thermal radiation. Hence, an exchange process is complicated by radiation penetrating into and coming out of material. The radiation leaving an area depends on the temperature distribution inside that area and that is unknown and is affected by the exchange process to other areas. The analysis has allowed for unlimited spectral detail but assumes that the various material properties do not vary significantly with temperature. Transient temperature distributions are obtained for the boundary conditions of external radiation and convection. The present analysis includes the influence of reflectivity, surface radiative properties and spectral properties on the temperature distributions.     

Meshless approach for coupled radiative and conductive heat transfer in one-dimensional graded index medium

A meshless local Petrov-Galerkin (MLPG) approach is employed for solving the coupled radiative and conductive heat transfer in a one-dimensional slab with graded index media. The angular distribution term in discrete ordinate equation of radiative transfer within a one-dimensional graded index slab is discretized by a step scheme, and the meshless approach for radiative transfer is based on the discrete ordinate equation. A moving least-squares approximation is used to construct the shape function. Two particular test cases for coupled radiative and conductive heat transfer within a one-dimensional graded index slab are examined to verify this new approximate method. The temperatures and the radiative heat fluxes are obtained. The results are compared with the other benchmark approximate solutions. By comparison, the results show that the MLPG approach has a good accuracy in solving the coupled radiative and conductive heat transfer in one-dimensional graded index media.     

The primary design of advanced ground-based atmospheric microwave sounder and retrieval of physical parameters

This paper introduces a prototype of ground-based atmospheric microwave sounder that operates in K-band from 22 to 31 GHz and V-band from 51 to 59 GHz. Different from the MP3000A and RPG, the sounder adopts independent dual-band reflectors instead of sharing a dual-band reflector. The direct detect type receiver is applied, which is of smaller size, higher sensitivity, efficient data observing and lower nonlinear error than the widely used superheterodyne receiver. The observing brightness temperatures from this prototype agree well with the simulated brightness temperatures according to the ground-based radiative transfer theory. We use the artificial neural network (ANN) algorithm to retrieve temperature profiles, which has higher spatial resolution especially in the capping inversion when compared with the linear regression algorithm. The temperature retrievals are comparable with the retrievals from RPG and MP3000A retrieval models and have a smaller bias in some certain regions.     

A numerical scheme for the solution of axisymmetric radiative transfer problems in irregular domains filled by media with opaque and transparent diffuse and specular boundaries

This paper presents a new numerical scheme of the discrete ordinates method for the solution of axisymmetric radiative transfer problems in irregular domains filled by media with opaque and transparent diffuse and specular (Fresnel) boundaries and interfaces. New test problems of radiative transfer, which describe radiative transfer in domains with Fresnel interfaces, are proposed in this paper. These problems admit analytic solutions and can be used as benchmark ones. The proposed scheme is applied to the solution of the problems. Numerical results show that the presence of Fresnel interfaces leads to an appreciably larger error in numerical solution. This is connected with the “discontinuity” of the Fresnel reflectivity, which, through numerical diffusion, leads to the distortion of numerical solution. Modification of the scheme allows to reduce the numerical error.     

Temperature Diagnostics of a Z-Pinch Plasma Using Calculations of the Spectral Brightness of X-Ray Radiation in a Large Interval of Radiation Energies

We elaborate a modern approach to the temperature diagnostics of a Z-pinch plasma. The approach is based on quantum-mechanical calculations of spectral brightness for X-ray radiation performed in a large interval of the photon energy for several temperatures and densities. In a large interval of the photon energy, a range can be found where the spectral brightness is highly sensitive to the temperature variation. This fact enables temperature diagnostics without complicated analysis of the spectral-line shape used in traditional diagnostic methods. In our calculations of the spectral brightness of X-ray radiation, we use a theoretical model known as the ion model of a plasma. We discuss important features of this model along with the other theoretical models used for calculating the radiative properties of the plasma. We calculate the spectral brightness of X-ray radiation for molybdenum plasma at temperatures of 1 and 1.2 keV and plasma densities of 1 and 2 g/cm³ and find the range of X-ray radiation energies that can be used for the temperature diagnostics.     

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.     

普朗克权重订正对风云卫星快速正演精度的改进

本文在逐线积分(LBL)计算卷积大气透过率的基础上,建立了统一框架的快速透过率回归算法用于卫星红外传感器通道辐射快速正演的实现.通过分别对风云二号自旋扫描辐射计VISSR/FY-2F和风云三号红外分光计IRAS/FY-3B的红外通道进行正演,并与LBL积分的通道亮温进行对比分析.结果表明,快速正演模型在窗区红外通道亮温的标准差(STD)远大于垂直探测通道的正演,随着通道探测高度降低,正演偏差随之增大;并且,全部通道正演的STD最大值小于0.4 K.本文还讨论了在卷积通道透过率时引入普朗克函数作为权重因子以改进红外通道的快速正演精度.提出以2350 cm-1与通道光谱响应曲线的相交作为应用启动普朗克订正的必要条件.试验结果表明:普朗克权重订正可以使快速正演模型的精度提高约25%以上,这个订正仅对通道光谱响应与2350 cm-1相交的通道有效,当通道光谱响应与2350 cm-1不相交,普朗克订正效果明显下降.试验还表明,普朗克订正的效果不仅对宽光谱的成像通道有效,对窄光谱垂直探测通道的正演精度改进同样明显.     

A fast infrared radiative transfer model for overlapping clouds

A fast infrared radiative transfer model (FIRTM2) appropriate for application to both single-layered and overlapping cloud situations is developed for simulating the outgoing infrared spectral radiance at the top of the atmosphere (TOA). In FIRTM2 a pre-computed library of cloud reflectance and transmittance values is employed to account for one or two cloud layers, whereas the background atmospheric optical thickness due to gaseous absorption can be computed from a clear-sky radiative transfer model. FIRTM2 is applicable to three atmospheric conditions: (1) clear-sky, (2) single-layered ice or water cloud, and (3) two simultaneous cloud layers in a column (e.g., ice cloud overlying water cloud). Moreover, FIRTM2 outputs the derivatives (i.e., Jacobians) of the TOA brightness temperature with respect to cloud optical thickness and effective particle size. Sensitivity analyses have been carried out to assess the performance of FIRTM2 for two spectral regions, namely the longwave (LW) band (587.3-1179.5 cm⁻¹) and the short-to-medium wave (SMW) band (1180.1-2228.9 cm⁻¹). The assessment is carried out in terms of brightness temperature differences (BTD) between FIRTM2 and the well-known discrete ordinates radiative transfer model (DISORT), henceforth referred to as BTD (F−D). The BTD (F−D) values for single-layered clouds are generally less than 0.8 K. For the case of two cloud layers (specifically ice cloud over water cloud), the BTD (F−D) values are also generally less than 0.8 K except for the SMW band for the case of a very high altitude (>15 km) cloud comprised of small ice particles. Note that for clear-sky atmospheres, FIRTM2 reduces to the clear-sky radiative transfer model that is incorporated into FIRTM2, and the errors in this case are essentially those of the clear-sky radiative transfer model.     

A numerical study on the contribution of melting layer composed by coated ice spheres to the upwelling brightness temperatures in TMI channels using VDISORT algorithm

To examine the influence of bright band on the retrieval of precipitation rate, the performance of melting layer composed by coated ice precipitable particles on the satellite-based measurement of polarized microwave brightness temperatures is discussed in this article by a vector discrete ordinate radiative transfer model. After comparing the simulated brightness temperatures in different TMI channels with and without the melting layer, we conclude that: (1) The melting layer composed by liquid-coated ice spheres weakens the upwelling microwave brightness temperatures because of the absorption/emission effect caused by the liquid coat. This effect is more conspicuous in middle and high frequency channels (19, 37 and 85 GHz) but, in 85 GHz channel, with the increase of rain rate, the multi-scattering can weaken its effect. (2) In a specific frequency, the horizontally polarized brightness temperature is more severely weakened by the melting layer than the vertically polarized. With the “cold” background (ocean surface, for example), this character is more conspicuous than that with a warm background. That is to say, the inner structure of a cloud system is easier to be detected under a cold background. Only in the 85 GHz frequency and when the rain rate is larger than 4 mm/h can we find that the vertically polarized brightness temperature is more severely weakened than the horizontally polarized one. (3) The melting layer with the assumption of coated ice spheres can change the difference of brightness temperatures between the vertically and horizontally polarized channels in the same frequency. In general, the value of such difference with the assumption of melting layer is larger than that without it. With a warm background, this value is negative and only in middle frequency (37 GHz), it is both stable and conspicuous.     

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.     

太阳能吸热器玻璃窗辐射与导热计算

在太阳能塔式热发电空气吸热器工作过程中,吸热器玻璃窗受到聚焦太阳光、环境以及吸热面（体）的复合辐射。本文采用改进蒙特卡罗法、有限容积法,研究了太阳能空气吸热器石英玻璃窗半透明介质内部辐射与导热的耦合换热,研究了不同厚度半透明介质石英玻璃窗在工作面处于不同第三类边界条件下的稳态温度分布。研究表明石英玻璃的厚度和内侧空气温...     

星载极化相关型全极化微波辐射计天线交叉极化校正技术 (II) : 校正试验

创建了地球场景数据集, 结合全极化微波辐射传输模型, 仿真了地球场景亮温. 基于自主推导的全极化天线温度方程, 通过GRASP9软件生成天线方向图, 模拟了辐射计的天线温度. 进而利用多元线性回归方法, 求取了天线交叉极化校正M矩阵, 实现了对星载极化相关型全极化微波辐射计天线交叉极化的校正. 试验结果表明: 天线温度与地球场景亮温之间具有良好的线性关系; 天线交叉极化对全极化微波辐射计正交通道亮温影响明显, 尤其以对垂直极化亮温误差的影响最为显著; 校正后各通道的天线交叉极化得到了有效的减小, 交叉极化优于-23 dB, 极化纯度大于99.5%, 采用M矩阵校正及消除天线温度中交叉极化亮温影响的方案是切实可行的. 该校正技术可以实现星载极化相关型全极化微波辐射计在轨运行后对于天线交叉极化的最终校正. 关键词： 全极化微波辐射计 天线交叉极化 天线温度方程 M矩阵')" href="#">M矩阵     

High-Precision Direct Method for the Radiative Transfer Problems

It is the main aim of this paper to investigate the numerical methods of the radiative transfer equation.Using the five-point formula to approximate the differential part and the Simpson formula to substitute for integral part respectively, a new high-precision numerical scheme, which has 4-order local truncation error, is obtained. Subsequently,a numerical example for radiative transfer equation is carried out, and the calculation results show that the new numerical scheme is more accurate.     

Radiative transfer solutions for coupled atmosphere ocean systems using the matrix operator technique

Accurate radiative transfer models are the key tools for the understanding of radiative transfer processes in the atmosphere and ocean, and for the development of remote sensing algorithms. The widely used scalar approximation of radiative transfer can lead to errors in calculated top of atmosphere radiances. We show results with errors in the order of±8% for atmosphere ocean systems with case one waters. Variations in sea water salinity and temperature can lead to variations in the signal of similar magnitude. Therefore, we enhanced our scalar radiative transfer model MOMO, which is in use at Freie Universität Berlin, to treat these effects as accurately as possible. We describe our one-dimensional vector radiative transfer model for an atmosphere ocean system with a rough interface. We describe the matrix operator scheme and the bio-optical model for case one waters. We discuss some effects of neglecting polarization in radiative transfer calculations and effects of salinity changes for top of atmosphere radiances. Results are shown for the channels of the satellite instruments MERIS and OLCI from 412.5 nm to 900 nm.