Cloud-induced effects on monthly averaged scintillation amplitude along millimeter-wave slant paths

A physical-statistical analysis of cloud-induced effects on millimeter-wave amplitude scintillation along Earth-satellite slant paths is presented. Using numerical simulations derived from a cloud radiative model, a dual-channel nonlinear retrieval algorithm is set up to estimate simultaneously cloud columnar water vapor and liquid water contents from measured ground-based brightness temperatures. The latter are related in cloudy conditions to surface meteorological variables and columnar water vapor and liquid water contents. ITALSAT ground-station data consisting of time series of beacon scintillation at 18.7, 39.6, and 49.5 GHz, surface meteorological data and radiometric observations at 13.0, 23.8, and 31.6 GHz, are used for an experimental analysis of cloud-induced scintillation by selecting a period of six years from 1994 to 1999. The results show a significant correlation between amplitude scintillation variances and cloud columnar contents on a monthly basis (i.e., averaged on-time intervals of one month). Considerations about possible impact of this analysis on the development of scintillation prediction models are finally illustrated.     

联合CloudSat-CALIPSO-MODIS进行云相态检测的新方法(英文)

云与生活息息相关,云参量的定量研究便显得极其重要 ,其中包括云相态的精确判识。由于传统基于单传感器的云相态识别算法都存在一定的局限性,提出了联合CloudSat-CALIPSO-MODIS多传感器进行云相态检测的新方法,提高了云相态的识别精度。利用2008年5月2日和2010年2月1日的CloudSat、CALIPSO、MODIS综合观测数据,获取了6种云相态,包括不确定云,混合云, 水云,过冷水云,冰云和晴空。结果表明协同算法可以更精确地进行云相态识别,并为数值天气预报提供条件,具有重要的科学意义。     

A nonlinear optimization algorithm for WindSat wind vector retrievals

WindSat is a space-based polarimetric microwave radiometer designed to demonstrate the capability to measure the ocean surface wind vector using a radiometer. We describe a nonlinear iterative algorithm for simultaneous retrieval of sea surface temperature, columnar water vapor, columnar cloud liquid water, and the ocean surface wind vector from WindSat measurements. The algorithm uses a physically based forward model function for the WindSat brightness temperatures. Empirical corrections to the physically based model are discussed. We present evaluations of initial retrieval performance using a six-month dataset of WindSat measurements and collocated data from other satellites and a numerical weather model. We focus primarily on the application to wind vector retrievals.     

TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphererange correction algorithm and pre-launch error budget

For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted     

Infrared Radiance Simulation and Application under Cloudy Sky Conditions Based on HIRTM

An algorithm based on hyperspectral infrared cloudy radiative transfer model (HIRTM) is introduced and a simulation method for infrared image of the generation geostationary meteorological satellite is proposed. Based on the parameters from weather research and forecast (WRF), such as the water content, atmospheric temperature, and humidity profile, the simulation data for the advanced Himawari imager (AHI) infrared radiative (IR) channels of Himawari-8 are obtained. Simulated results based on HIRTM agree well with the observed data. Further, the movement, development, and change of the cloud are well predicated. And the simulation of IR cloud image for the weather forecast has been obtained. This paper provides an improved method for evaluation and improvement of regional numerical model for weather forecast.     

Semivectorial Helmholtz beam propagation by Lanczos reduction

By using the finite-difference method to perform Lanczos reduction, a semivectorial Helmholtz beam propagation algorithm is demonstrated. The applicability of this algorithm is no longer limited to paraxial beams and scalar fields. Mode indices of rib waveguides are calculated and compared to previously published data. Losses of Y -branch for two orthogonal polarizations are also presented. This algorithm is more efficient than the conventional beam propagation method in determining the mode index. To calculate the radiation loss however, it requires much more computational effort. More than 30 Krylov vectors are needed to avoid numerical dissipation     

Numerical sensitivity analysis of passive EHF and SMMW channels totropospheric water vapor, clouds, and precipitation

Potential uses of specific extremely high frequency (EHF) and submillimeter-wave (SMMW) channels at 90, 166, 283, 220, 325, 340, and 410 GHz for passive spaceborne remote sensing of the troposphere and lower stratosphere are investigated using an iterative numerical radiative transfer model. Collectively, these channels offer potential for high spatial resolution imaging using diffraction-limited apertures of practical size, along with the ability to profile water vapor, map precipitation beneath optically opaque cloud cover, and to measure nonprecipitating cloud (e.g. cirrus) parameters. A passive airborne imaging instrument for tropospheric meteorological sensing at 90, 150, 183±1, 3, 7, 220, and 325±1, 3, 9 GHz, called the Millimeter-wave Imaging Radiometer (MIR), is described     

A new recombination model for device simulation including tunneling

A recombination model for device simulation that includes both trap-assisted tunneling (under forward and reverse bias) and band-to-band tunneling (Zener tunneling) is presented. The model is formulated in terms of analytical functions of local variables, which makes it easy to implement in a numerical device simulator. The trap-assisted tunneling effect is described by an expression that for weak electric fields reduces to the conventional Shockley-Read-Hall (SRH) expression for recombination via traps. Compared to the conventional SRH expression, the model has one extra physical parameter, the effective mass m*. For m*=0.25 m₀ the model correctly describes the experimental observations associated with tunneling. The band-to-band tunneling contribution is found to be important at room temperature for electric fields larger than 7×10⁵ V/cm. For dopant concentrations above 5×10¹⁷ cm^-3 or, equivalently, for breakdown voltages below approximately 5 V, the reverse characteristics are dominated by band-to-band tunneling     

Simple I/V model for short-channel i.g.f.e.t.s in the triode region

The standard 1-dimensional model for an i.g.f.e.t. in the triode region predicts values of the drain current ID that fall increasingly below experimental values as the channel is shortened or as the drain voltage VD is increased. However, a strictly 2-dimensional treatment results in prohibitive computation. A model is described that takes into account fringing fields at each end of the channel and which requires a relatively simple numerical solution of two nonlinear equations. Experiments on p channel i.g.f.e.t.s with gate lengths of 1.6 to 7.8 ?m show that this model predicts I/V characteristics much better than does the standard 1-dimensional model which includes the bulk charge.     

One-dimensional variational retrieval algorithm of temperature, water vapor, and cloud water profiles from advanced microwave sounding unit (AMSU)

The measurements from satellite microwave imaging and sounding channels are simultaneously utilized through a one-dimensional (1-D) variation method (1D-var) to retrieve the profiles of atmospheric temperature, water vapor and cloud water. Since the radiative transfer model in this 1D-var procedure includes scattering and emission from the earth's atmosphere, the retrieval can perform well under all weather conditions. The iterative procedure is optimized to minimize computational demands and to achieve better accuracy. At first, the profiles of temperature, water vapor, and cloud liquid water are derived using only the AMSU-A measurements at frequencies less than 60 GHz. The second step is to retrieve rain and ice water using the AMSU-B measurements at 89 and 150 GHz. Finally, all AMSU-A/B sounding channels at 50-60 and 183 GHz are utilized to further refine the profiles of temperature and water vapor while the profiles of cloud, rain, and ice water contents are constrained to those previously derived. It is shown that the radiative transfer model including multiple scattering from clouds and precipitation can significantly improve the accuracy for retrieving temperature, moisture and cloud water. In hurricane conditions, an emission-based radiative transfer model tends to produce unrealistic temperature anomalies throughout the atmosphere. With a scattering-based radiative transfer model, the derived temperature profiles agree well with those observed from aircraft dropsondes.     

An empirical algorithm for cloud liquid water from MSMR and its utilization in rain identification

In this paper, an empirical method to estimate cloud liquid water from Indian Remote Sensing P4 (IRS-P4) Multi-frequency Scanning Microwave Radiometer (MSMR) measurements is presented. MSMR brightness temperatures are collocated with concurrent observations of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI)-derived cloud liquid water. The multiple-correlation coefficient between TMI-derived cloud liquid water and logarithmic of MSMR-derived brightness temperatures, and their differences at 18- and 21-GHz channels, is found to be about 82.4%. The relationship thus obtained has an rms error of 8.75 mgcm/sup -2/ in the measurements of cloud liquid water from MSMR with respect to TMI measurements. Verification of the algorithm is carried out with another set of concurrent measurements from MSMR and TMI. Further, the MSMR-derived cloud liquid water over the global oceans and for extreme weather conditions (cyclone) are compared with that from TMI and the Special Sensor Microwave/Imager (SSM/I) for independent verification. The cloud liquid water from MSMR is further used to successfully delineate rain events for quantitative estimation of rain rate from MSMR.     

Optimization of signal sets for partial-response channels. I.Numerical techniques

Given a linear, time-invariant, discrete-time channel, the problem of constructing N input signals of finite length K that maximize minimum l₂ distance between pairs of outputs is considered. Two constraints on the input signals are considered: a power constraint on each of the N inputs (hard constraint) and an average power constraint over the entire set of inputs (soft constraint). The hard constraint, problem is equivalent to packing N points in an ellipsoid in min(K,N-1) dimensions to maximize the minimum Euclidean distance between pairs of points. Gradient-based numerical algorithms and a constructive technique based on dense lattices are used to find locally optimal solutions to the preceding signal design problems. Two numerical examples are shown for which the average spectrum of an optimized signal set resembles the water pouring spectrum that achieves Shannon capacity, assuming additive white Gaussian noise     

基于集成学习的风云四号遥感图像云相态分类算法

云相态分类在气象预报和气候研究中具有重要的地位。我国新一代气象卫星风云四号的成像仪在光谱通道数量和空间分辨率较上一代风云二号有较大提升,这为云相态的研究提供了新的遥感数据。本文首先对风云四号相隔15 min的遥感图像进行分析,然后提出亮温云相态指数,该指数可以进行初步云相态分类,最后在此基础上提出基于集成学习的云相态分类算法。实验结果与风云四号官方云相态分类结果进行比较,水云的一致率达到91.69%,冰云的一致率达到76.10%。     

Finite-volume time-domain (FVTD) techniques for EM scattering

The authors document our efforts to compute radar cross sections using a generalization of the FDTD (finite difference time domain) Yee algorithm (1966) in which a nonorthogonal target-conforming mesh may be applied. Both 2D and 3D formulations are presented, although at present they only have 2D numerical results. Two-dimensional problems considered involve an S-shaped duct (S-duct) and an ogive. At least nine distinct resonances are correctly predicted by the S-duct model. The primary goal is to model oblique or curved surfaces without the use of staircasing     

Retrieval of Atmospheric Integrated Water Vapor and Cloud Liquid Water Content Over the Ocean From Satellite Data Using the 1-D-Var Ice Cloud Microphysics Data Assimilation System (IMDAS)

Reliable prediction of precipitation by numerical weather prediction (NWP) models depends on the appropriate representation of cloud microphysical processes and accurate initial conditions of observations of atmospheric variables. Therefore, to retrieve reasonable cloud distributions, a 1-D variational Ice Cloud Microphysics Data Assimilation System (IMDAS) is developed to improve the predictability of NWP models. The general framework of IMDAS includes the Lin ice cloud microphysics scheme as a model operator, a four-stream fast microwave radiative transfer model in the atmosphere as an observation operator, and a global minimization method that is known as the shuffled complex evolution. IMDAS assimilates the satellite microwave radiometer data set of the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) and retrieves integrated water vapor and integrated cloud liquid water content. This new method successfully introduces heterogeneity into the initial state of the atmosphere, and the modeled microwave brightness temperatures agree well with the observations of the Wakasa Bay Experiment 2003 in Japan. It has significantly improved the performance of the cloud microphysics scheme by the intrusion of heterogeneity into the external global reanalysis data, which resultantly improved atmospheric initial conditions.     

An adaptive clustering algorithm for image segmentation

The problem of segmenting images of objects with smooth surfaces is considered. The algorithm that is presented is a generalization of the K-means clustering algorithm to include spatial constraints and to account for local intensity variations in the image. Spatial constraints are included by the use of a Gibbs random field model. Local intensity variations are accounted for in an iterative procedure involving averaging over a sliding window whose size decreases as the algorithm progresses. Results with an 8-neighbor Gibbs random field model applied to pictures of industrial objects, buildings, aerial photographs, optical characters, and faces show that the algorithm performs better than the K-means algorithm and its nonadaptive extensions that incorporate spatial constraints by the use of Gibbs random fields. A hierarchical implementation is also presented that results in better performance and faster speed of execution. The segmented images are caricatures of the originals which preserve the most significant features, while removing unimportant details. They can be used in image recognition and as crude representations of the image     

Water vapor profiling over ocean surface from airborne 90 and 183GHz radiometric measurements under clear and cloudy conditions

Radiometric measurements at 90 GHz and three sideband frequencies near the water vapor absorption line at 183.3 GHz were made with the Advanced Microwave Moisture Sounder (AMMS) aboard the NASA DC-8 aircraft over some regions of the Pacific Ocean during November 1989. These measurements were used to retrieve atmospheric water vapor profiles over ocean surface using the algorithm developed by T.T. Wilheit (1979). The algorithm incorporates a mechanism to estimate cloud liquid water when the estimated relative humidity is greater than 95%. The results are compared with the estimated values from the measurements of Special Sensor Microwave Imager (SSMI) and TIROS Operational Vertical Sounder (TOVS). The water vapor profiles estimated from AMMS are generally higher at low altitudes and lower at high altitudes compared to those from the TOVS measurements. Values of total precipitable water estimated from the AMMS and SSM/I are in general agreement. Cloud liquid water vapor profiles retrieved from the AMMS show more fluctuations than those from SSM/I     

Medium earth orbit optical satellite communication networks: Ground terminals selection optimization based on the cloud‐free line‐of‐sight statistics

In this paper, a methodology for the generation of cloud coverage time series correlated on both temporal and spatial domains and the estimation of cloud‐free line‐of‐sight (CFLOS) probability for a single optical ground station (OGS) and a network of OGSs (multiple OGS) for medium earth orbit (MEO) constellation satellite communication systems is presented. Spatial diversity is employed as a mitigation technique against cloud blockage to increase the availability of an OGS network (OGSN). In the second part of the paper, an effective algorithm based on the ant colony optimization (ACO) algorithm is proposed for the optimum selection of OGSs forming an OGSN. The proposed algorithm takes into consideration the spatial correlation between the OGSs. In the simulation results section, a MEO constellation system with 12 MEO satellites has been assumed. The integrated liquid water content (ILWC) statistical parameters that are necessary for the CFLOS time series are taken from ECMWF Re‐Analysis (ERA)‐Interim database, European Centre for Medium‐Range Weather Forecasts (ECMWF). Finally, numerical results for the optimum selection of OGSs using the proposed methodology are exhibited and commented. Interesting conclusions are drawn, and future work with technical challenges is briefly described.     

Computer simulation of ESD from voluminous objects compared totransient fields of humans

Computer simulation of electrostatic discharge (ESD) for simplified objects is described and compared to measured transient fields of human/metal ESD. The simulation algorithm uses the method of moments in time domain, coupled with nonlinear arc resistance model. Transient currents and fields are analyzed from the electromagnetic compatibility (EMC) point of view. Validation of the numerical simulation is done by comparison to experimental data. The simulated structure models the human/metal ESD in its peak current and field values and their derivatives reasonably well     

A fast algorithm for the Fourier transform over finite fields andits VLSI implementation

The Fourier transform over finite fields is mainly required in the encoding and decoding of Reed-Solomon and BCH codes. An algorithm for computing the Fourier transform over any finite field GF(p^m) is introduced. It requires only O(n(log n)²/4) additions and the same number of multiplications for an n-point transform and allows in some fields a further reduction of the number of multiplications to O(n log n). Because of its highly regular structure, this algorithm can be easily implementation by VLSI technology