Measurements and predictions of the RCS of Bruderhedrals atmillimeter wavelengths

Polarimetric radar cross sections (RCSs) were measured for two large Bruderhedrals at 35 and 93 GHz. The overall spread in the measured RCS was 2 dB. It was found that the absolute RCS as well as the dependence of the RCS on elevation angle could be significantly altered by a slight misalignment of the Bruderhedral. The calculations of these RCSs by theories for a top hat and RCS simulation codes based on geometric and physical optics (PO) generally agreed with the measurements to within 3 db. Radar calibrations using these Bruderhedrals that are based on the predictions are not reliable, however, if accuracies better than 3 dB are required, The measured values of the cross-polarization RCS were very small compared to the copolarization ones, consistent with the codes and theoretical considerations     

Polarimetric backscatter from fresh and metamorphic snowcover atmillimeter wavelengths

This paper presents 35, 95, and 225 GHz polarimetric radar backscatter data from snowcover. It compares measured backscatter data with detailed in situ measurements of the snowcover including microstructural anisotropies within the snowpack. Observations of backscatter mere made during melt-freeze cycles, and measurable differences in the normalized radar cross section between older metamorphic snow and fresh low-density snow were observed. In addition, these data show that the average phase difference between the copolarized terms of the scattering matrix, S_vvand S_hh , is nonzero for certain snow types. This phase difference was found to be related to snowpack features including anisotropy, wetness, density, and particle size. A simple backscatter model based on measured particle size and anisotropy is found to predict the Mueller matrix for dry snowcover with reasonable accuracy     

Cross-polarized radar reflections from wet snow and ice droplets atweather radar wavelengths

The copolar radar reflection cross sections Q_HH and Q_VV from a collection of spheroid scatterers in the melting layer formed by ice precipitation prior to becoming rain have been analyzed by the authors in a recent work (IEEE Trans. Antennas and Propagation, vol.38, p.1317-25, Sept. 1990). The model is extended to calculate the cross-polarized cross section Q_VH due to reception at an electric-field polarization orthogonal to that of transmission. The authors are specifically interested in the cross-polarized differential reflectivity L_dr=10 log₁₀(Q_VH/Q_HH)     

Analysis and nonlinear simulation of a quadrupole wiggler FEL atmillimeter wavelengths

The use of a helical quadrupole wiggler field in a circular guide for amplification of millimeter waves is discussed. The effects of space charge and axial field are considered for beam stability for near-axis orbits. When space charge is neglected, the orbit equations and properties and characteristic betatron oscillation of frequencies are examined by a linearization of the orbit equations. A nonlinear simulation code for dipole wigglers is modified to examine optimum gain TE₂₁ waveguide modes for a quadrupole wiggler in the 30-300 GHz range     

Airborne SAR observations of ocean surface waves penetratingfloating ice

The results are presented for a new and improved procedure for estimating the synthetic-aperture radar (SAR) image spectrum of ocean waves. This procedure, the spectral-sum method, involves summing individual image spectra derived from each of the looks of a multilook set. An automatic registration of the per-look spectral information is achieved, accounting for subimage look misregistration due to the wave propagation between looks. Spectral-sum processing is compared with traditional look-sum processing as a function of the radar slant-range. Spectral-sum processing is applied to SAR imagery of waves penetrating the marginal ice zone     

Sensitivity of passive microwave snow depth retrievals to weather effects and snow evolution

Snow fall and snow accumulation are key climate parameters due to the snow's high albedo, its thermal insulation, and its importance to the global water cycle. Satellite passive microwave radiometers currently provide the only means for the retrieval of snow depth and/or snow water equivalent (SWE) over land as well as over sea ice from space. All algorithms make use of the frequency-dependent amount of scattering of snow over a high-emissivity surface. Specifically, the difference between 37- and 19-GHz brightness temperatures is used to determine the depth of the snow or the SWE. With the availability of the Advanced Microwave Scanning Radiometer (AMSR-E) on the National Aeronautics and Space Administration's Earth Observing System Aqua satellite (launched in May 2002), a wider range of frequencies can be utilized. In this study we investigate, using model simulations, how snow depth retrievals are affected by the evolution of the physical properties of the snow (mainly grain size growth and densification), how they are affected by variations in atmospheric conditions and, finally, how the additional channels may help to reduce errors in passive microwave snow retrievals. The sensitivity of snow depth retrievals to atmospheric water vapor is confirmed through the comparison with precipitable water retrievals from the National Oceanic and Atmospheric Administration's Advanced Microwave Sounding Unit (AMSU-B). The results suggest that a combination of the 10-, 19-, 37-, and 89-GHz channels may significantly improve retrieval accuracy. Additionally, the development of a multisensor algorithm utilizing AMSR-E and AMSU-B data may help to obtain weather-corrected snow retrievals.     

一种表面发射率的测量方法研究

大型实验设备由于表面温度难以控制而使其表面发射率精确测量较为困难。在分析表面发射率测量误差主要来源的基础上,提出一种表面发射率工程测量方法。分析并推导了到达探测器上的辐射量,利用两发射率已知的设备,实现了环境辐射量的测量,从而消除了环境辐射对表面发射率测量的影响,有效提高了测量精度。设计了实验方法并进行了发射率测量实验,实验结果表明未考虑环境辐射的测量方法与该方法的计算结果相对误差可达20.37%,验证了该测量方法的可行性和准确性。     

A recurrent neural network classifier for improved retrievals ofareal extent of snow cover

Accurate detection of areal extent of snow in mountainous regions is important. Areal extent of snow is a useful climatic indicator. Moreover, snow melt is a major source of water supply for many arid regions (e.g., western United States, Morocco) and affects regional ecosystems. Unfortunately, accurate satellite retrievals of areal extent of snow have been difficult to achieve. Two approaches to effectively and accurately detect clear land, cloud, and areal extent of snow in satellite data are developed. A feed-forward neural network (FFNN) is used to classify individual images, and a recurrent NN is used to classify sequences of images. The continuous outputs of the NN, combined with a linear mixing model, provide support for mixed-pixel classification. Validation with independent in situ data confirms the classification accuracy (94% for feed-forward NN, 97% for recurrent NN). The combination of rapid temporal sampling (e.g., GOES) and a recurrent NN classifier is recommended (relative to an isolated scene (e.g., AVHRR) and a feed-forward NN classifier)     

Airborne measurements of forest and agricultural land surfaceemissivity at millimeter wavelengths

Passive microwave radiometers have been operated on an aircraft over the an area of Sweden, near Uppsala in September 1995 and March 1997 as part of the Northern Processes Experiment (NOPEX). Their measurements have allowed the calculation of the emissivity of boreal forest and agricultural land surfaces at 24, 50, 89, and 157 GHz over a range of incidence angles and polarizations. These results show consistent differences between dense forestry, where the emissivity is close to 1 and open land, where it is approximately 0.96. These differences are examined and a model is presented to parameterize these surfaces by use of a Debye-like effective permittivity and Fresnel's reflection coefficients. This will allow retrievals of atmospheric temperature and humidity profiles to be made by satellite microwave sounders, such as the advanced microwave sounding unit (AMSU) over similar land surfaces     

Sea ice concentration, ice temperature, and snow depth using AMSR-E data

A summary of the theoretical basis and initial performance of the algorithms that are used to derive sea ice concentration, ice temperature, and snow depth on sea ice from newly acquired Earth Observing System-Aqua/Advanced Microwave Scanning Radiometer-EOS (AMSR-E) radiances is presented. The algorithms have been developed and tested using historical satellite passive microwave data and are expected to provide more accurate products, since they are designed to take advantage of the wider range of frequencies and higher spatial resolution of the AMSR-E microwave instrument. Validation programs involving coordinated satellite, aircraft, and surface measurements to determine the accuracies of these sea ice products and to improve further our capability to monitor global sea ice are currently underway.     

Snow cover detected by diurnal warming of sea ice/snow surface offLabrador in NOAA imagery

Four consecutive NOAA-9 infrared images show diurnal warming of the sea-ice surface off the coast of Labrador. For three subdomains with scales of ~100 km, the warming of the surface is obtained on the assumption that the warming of the surface is obtained on the assumption that the warming in one pixel is proportional to ice compactness, which is given in turn by brightness of a visible (daytime) image. A one-dimensional heat transfer equation for snow and ice is solved. An algorithm to estimate the snow thickness, which is required to calculate the heat flux through ice, from this extra warming is proposed     

Effective permittivity of and scattering from wet snow and icedroplets at weather radar wavelengths

In this parametric study, wet snow and ice droplets are modeled as sparse collections of Rayleigh scatterers (size small compared to wavelength) consisting either of ice or of composite mixtures of air and ice in water. An effective permittivity is calculated using various extended Maxwell-Garnett-type models to account for variations in shape and orientation of the constituents. The backscatter radar cross section is calculated as an incoherent sum of individual particle cross sections, and for various distributions of shape, size, and orientation. The results indicate a dependence of the radar cross section on the polarizations of the incident and reflected fields. This dependence is shown in the differential reflectivity, defined in terms of the ratio of the backscatter cross sections due to two mutually orthogonal linearly polarized incident electric fields     

Backscatter from snow and ice surfaces at near incident angles

The radar backscatter of natural snow surfaces was measured at 10 GHz and 35 GHz and at grazing angles from1degto0.3deg. For horizontal polarized radiation the terrain clutter per unit area (m²) at 10 GHz of a flat snow terrain decreases from -50 dB at1degto -70 dB at0.4deg. The return is approximately 10 dB lower for vertical polarized radiation. The terrain clutter was found to depend on the free water content of the snow. The radar cross sections of ice blocks placed on the snow surface is roughly proportional to the square of the area of the ice block facing the radar at 10 and 35 GHz and is approximately 20 dBsm below the return expected for a perfectly reflecting plane surface. At 95 GHz the ice blocks become diffuse reflectors.     

基于ASTER数据的地物光谱比辐射率的反演研究

比辐射率光谱表征着一个物体的内部物理和化学特征,是定量化遥感的一个关键参数.本文利用ASTER数据的第10至14波段,根据其数据的特点,基于温度比辐射率分离算法的思想,提出了将比辐射率标准化法(Normalize Emissivity Method,NEM)、经验公式、比值法(Ratio Method)这几个模块结合起来,在迭代的基础上计算出比辐射率的新算法.本文简要分析了模型误差的主要来源,并且把本文的算法与简化的ASTER的TES算法进行了比较.分析表明本文的算法是可行、有效的,而且该算法简单,易于实现,在能够保证精度的情况下运算速度快;同时也说明ASTER遥感数据用于反演地物的比辐射率可以得到比较理想的结果,具有良好的应用前景.     

Nonlinear statistical retrievals of ice content and rain rate frompassive microwave observations of a simulated convective storm

A numerical simulator for analysis of multispectral passive microwave mapping and retrieval is described. This simulator allows evaluation and optimization of satellite-based cloud and precipitation parameter retrieval algorithms. It contains three major components: the forward radiative transfer model, the sensor observation model, and the parameter retrieval algorithm. Simulated spaceborne observations of an oceanic tropical squall sampled at five stages in time are demonstrated for a simplified version of the proposed Earth Observation System (EOS) Multifrequency Imaging Microwave Radiometer (MIMR). The simulator uses a nonlinear statistical retrieval algorithm consisting of a Karhunen-Loeve (KL) transform, a projection operator, a nonlinear inverse mapping and a linear minimum mean-square error estimator. Retrievals of rain rate and integrated ice content are performed for each evolutionary frame at both full spatial resolution (1.5 km) and the degraded spatial resolution of a MIMR-class system. Results are presented for both KL-based and brightness temperature-based retrieval algorithms. It is found that the KL-based algorithm has a reduced complexity and performs better than the brightness temperature-based algorithm for degraded resolution imagery, especially for rain rate retrievals. In addition, rain rate retrievals are more affected by low image resolution than are integrated ice content retrievals. Retrieval accuracy of both rain and integrated ice is also found to depend on the evolutionary stage of the storm     

Comparison of passive microwave ice concentration algorithm retrievals with AVHRR imagery in arctic peripheral seas

An accurate representation of sea ice concentration is valuable to operational ice analyses, process studies, model inputs, and detection of long-term climate change. Passive microwave imagery, such as from the Special Sensor Microwave/Imager (SSM/I), are particularly valuable for monitoring of sea ice conditions because of their daily, basin-scale coverage under all sky conditions. SSM/I-derived sea ice concentration estimates using four common algorithms [Bootstrap (BT), Cal/Val (CV), NASA Team (NT), and NASA Team 2 (N2)] are compared with concentrations computed from Advanced Very High Resolution Radiometer (AVHRR) visible and infrared imagery. Comparisons are made over approximately an eight-month period in three regions of the Arctic and focus on areas near the ice edge where differences between the algorithms are likely to be most apparent. The results indicate that CV and N2 have the smallest mean error relative to AVHRR. CV tends to overestimate concentration, while the other three algorithms underestimate concentration. NT has the largest underestimation of nearly 10% on average and much higher in some instances. In most cases, mean errors of the SSM/I algorithm were significantly different from each other at the 95% significance level. The BT algorithm has the lowest error standard deviation, but none of the considered algorithms was found to have statistically significantly different error standard deviations in most cases. This indicates that spatial resolution is likely a limiting factor of SSM/I in regions near the ice edge in that none of the algorithms satisfactorily resolve mixed pixels. Statistical breakdowns by season, region, ice conditions, and AVHRR scene generally agree with the overall results. Representative case studies are presented to illustrate the statistical results.     

Ocean surface wind speed and direction retrievals from the SSM/I

A semiempirical model is developed that retrieves ocean surface wind direction information in addition to improved wind speeds from Special Sensor Microwave/Imager (SSM/I) measurements. Radiative transfer and neural network techniques were combined in the authors' approach. The model was trained and tested using clear sky cases, but atmospheric transmittance is retrieved so that retrieval in other than clear sky conditions is possible. With two SSM/I instruments currently providing operational ocean surface wind speed retrievals, the addition of wind direction information and improved wind speed retrievals will enhance the impact of this data in weather prediction models and marine weather forecasting     

深度连续渐变式空腔阵列表面发射率研究

为测量混合式表面的发射率,该文设计了一系列阵列作为发射率样品,发射率样品为深度连续渐变式空腔阵列,将其划分为5个单元组成,使用红外热像仪对发射率样品上端拍摄热图像得到一系列发射率。通过理论计算得出发射率,通过实验测量单元表面发射率,将理论计算值与实验测量值进行对比,探讨深度连续渐变式空腔阵列在不同加热温度下对表面发射率的影响。结果表明:随着加热温度的升高,深度连续渐变式空腔阵列的表面发射率会下降;随着空腔深度的增加,深度连续渐变式空腔阵列的表面发射率会随之增加。     

准球面模式下三种激光波长冰云大气反射率特性

冰云由冰晶粒子组成,是大气的重要组成部分,研究冰云条件下大气辐射传输特性有助于理解冰云对大气探测、地空链路激光通信等方面的作用。采用最新的C语言版本离散纵标法(CDISORT),比较了太阳天顶角不同时平面平行模式和准球面模式下冰云大气激光传输的反射率差异;数值计算准球面模式下三种激光波长(065μm,106μm,155μm)入射时冰云大气反射率随冰水含量、有效半径、光学厚度、云底高度、相对方位角等因素的变化情况。结果表明:当太阳天顶角小于70°时,两种模式下冰云大气激光传输的反射率相对误差很小;当太阳天顶角大于70°时,两种模式下冰云大气激光传输的反射率相对误差明显增大。冰云大气激光传输的反射率随有效半径的增大而减小,随光学厚度或冰水含量的增大而增大。三种不同的激光波长入射时,冰云大气的平均反照率差异较大,但均值大于08。本文结果为星地激光通信、卫星遥感等工程应用提供理论支撑。     

先进微波探测器资料反演地表微波辐射率试验

利用美国环境卫星A型先进微波探测器AMSU-A资料反演中国陆地区域地表微波辐射率.通过辐射传输正演模拟,提出了AMSU-A窗区通道反演地表微波辐射率的指数分析方法.利用模拟数据对比了指数分析方法和以往通道亮温组合方法.结果表明:对于地表比较干燥的地区,指教分析的反演结果略优于通道亮温组合的反演结果.在此基础上利用IGBP(International Geosphere-Biosphere Program)的地表分类数据,提取AMSU-A像元裸土组分的面积百分比信息,并进一步应用于AMSU-A像元裸土组分地表微波辐射率的反演试验,提高了反演精度,并可进一步应用于区域地表湿度信息提取和大气参数反演,以及AMSU-A窗区通道陆地区域遥感信息在数值预报模式中的同化应用.