用TOGA／COARE试验中机载雷达资料进行衰减效应订正的个案研究

为了蛎限度地利是生载测雨雷达，必须实现一些主要条件：高的空间分辨率、宽阔的覆盖面积，大的动态降雨测量范围以及精确的点测量。     

A dual-polarization rain profiling algorithm

A new attenuation correction algorithm based on profiles of reflectivity, differential reflectivity, and differential propagation phase shift is presented. A solution for specific attenuation retrieval in rain medium is proposed, which solves the integral equations for reflectivity and differential reflectivity with cumulative differential propagation phase shift constraint. The conventional rain profiling algorithms that connect reflectivity and specific attenuation can retrieve specific attenuation values along the radar path assuming a constant intercept parameter of the normalized drop size distribution. However, in convective storms, the drop size distribution parameters can have significant variation along the path. This paper presents a dual-polarization rain profiling algorithm for horizontal looking radars incorporating reflectivity as well as differential reflectivity profiles. The dual-polarization rain profiling algorithm has been evaluated with X-band radar observations simulated from drop size distribution derived from high-resolution S-band measurements collected by the Colorado Statue University CHILL radar. The analysis shows that the retrieved specific attenuation, differential attenuation, reflectivity, and differential reflectivity from the dual-polarization rain profiling algorithm provide significant improvement over the current algorithms.     

The influence of heavy rainfall on attenuation at 18.5 and 30.9 GHz

Percent-of-time distributions of rain-induced fades obtained on a 6.4-km path in New Jersey operating at a frequency of 18.5 GHz are discussed for the period of 1968-1969; data obtained at 30.9 GHz on a 1.9-km path for the same period are also discussed. The attenuation distributions are compared with attenuations calculated from the distributions of average rain rates on the paths. With these data, it is found that distributions of attenuation can be predicted from the path-average rain-rate distributions for a given sample period. Detailed rain-rate and attenuation measurements at 18.5 GHz on the 6.4-km path are presented for the most intense storm observed in a three year recording period. Point rain rates in excess of 250 mm/h and path-average rates exceeding 180 mm/h were observed; the attenuation exceeded the 50-dB dynamic measuring range of the equipment for more than seven minutes.     

Determination of Rain Rate from a Spaceborne Radar Using Measurements of Total Attenuation

Several experimental and theoretical studies have shown that path-integrated rain rates can be determined by means of a direct measurement of attenuation. For ground-based radars, this is done by measuring the backscattering cross section of a fixed target in the presence and absence of rain along the radar beam. A ratio of the two measurements yields a factor proportional to the attenuation from which the average rain rate can be deduced. In this paper, we extend the technique to spaceborne radars by choosing the ground as a reference target. The technique is also generalized so that both the average and range-profiled rain rates can be determined. The accuracies of the resulting estimates are evaluated for a narrow-beam radar located on a low earth-orbiting satellite.     

Retrieval of Snow and Rain From Combined X- and W-Band Airborne Radar Measurements

Two independent airborne dual-wavelength techniques, based on nadir measurements of radar reflectivity factors and Doppler velocities, respectively, are investigated with respect to their capability of estimating microphysical properties of hydrometeors. The data used to investigate the methods are taken from the ER-2 Doppler radar (X-band) and cloud radar system (W-band) airborne Doppler radars during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment campaign in 2002. Validity is assessed by the degree to which the methods produce consistent retrievals of the microphysics. For deriving snow parameters, the reflectivity-based technique has a clear advantage over the Doppler-velocity-based approach because of the large dynamic range in the dual- frequency ratio (DFR) with respect to the median diameter D₀ and the fact that the difference in mean Doppler velocity at the two frequencies, i.e., the differential Doppler velocity (DDV), in snow is small relative to the measurement errors and is often not uniquely related to D₀. The DFR and DDV can also be used to independently derive D₀ in rain. At W-band, the DFR-based algorithms are highly sensitive to attenuation from rain, cloud water, and water vapor. Thus, the retrieval algorithms depend on various assumptions regarding these components, whereas the DDV-based approach is unaffected by attenuation. In view of the difficulties and ambiguities associated with the attenuation correction at W-band, the DDV approach in rain is more straightforward and potentially more accurate than the DFR method.     

Rain attenuation statistics useful for the design of mobilesatellite communication systems

We have investigated how rain attenuation statistics, necessary to design fixed satellite systems working at frequencies greater than 10 GHz, are transformed to those applicable to the design of mobile satellite systems working in the same frequency bands and weather conditions in the special case of vehicles driven in zig-zag patterns to simulate city streets. The vehicles' speed has been modeled as a lognormal random variable, a mathematical model derived from measurements performed in freeways or in city traffic. We have used a large number of rain-rate time series collected in Italy (Gera Lario and Fucino) and in Canada (Montreal) to simulate rain-rate spatial fields and radio links at 19.77 GHz along a 30.6° slant path. The simulations have shown that a receiving or transmitting terminal moving in zig-zag patterns may experience, in the long term and for a given attenuation, a smaller outage probability, compared to the fixed terminal. For a given rain attenuation, the ratio between the outage probability of the mobile system and that of the fixed system (probability extrapolation factor ξ) is estimated to be around 0.2-0.3 in the optimistic case in which the probability of encountering rain is the same for all observers (fixed or in motion)-a less optimistic estimate shows that the probability of encountering rain might be twice as large and that, as a consequence, the values of ξ mentioned must be doubled. Conservative values of ξ can be calculated by using average values of rain storm speed and vehicles and average distances covered in the rain by fixed and mobile terminals. The results are less sensitive to changes in the geometrical or other parameters of the simulations     

Analysis of a Dual-Wavelength Surface Reference Radar Technique

Among the methods that have been proposed for estimating precipitation parameters from a down-looking radar are those that use the scattered power from the surface to infer path attenuation. In this paper, an analysis of a dual-wavelength surface reference technique (DSRT) is presented. The principal of the method is that the ratio of the backscattered power from the surface at one wavelength to that at a second wavelength is proportional to the differential attenuation of the signals through the intervening rain. To account for differences in the backscattering cross section of the surface a 0°, and in the radar calibration constant at the two wavelengths, measurements are made of the surface return power during clear air conditions. The characteristics of the method are best understood by comparing it to two closely related methods: the single-wavelength surface reference technique (SRT) and the standard dual-wavelength technique (DWT). A comparison between the DWT and the DSRT shows that the rain rate estimates are identical in form and that the surface reference can be obtained from the standard estimate by replacing the statistics of the rain reflectivity with those of ?°Advantages of the DSRT relative to its single wavelength counterpart occur when the wavelength correlation in ?° is high or when the mean values of ?° at the two wavelengths are nearly equal. To treat the methods under less restrictive assumptions a simulation is used. Error statistics of the rain rate estimates are presented for a space-borne radar operating at an attenuating wavelength.     

Improved range-profiling algorithm of rainfall rate from aspaceborne radar with path-integrated attenuation constraint

A range-profiling algorithm for rainfall rate retrieval from a single-frequency downward-looking spaceborne radar is presented. The algorithm is based on a linear reformulation of the radar equation. The path-integrated attenuation given by the surface echo measurement is used as a constraint for normalizing the range-gated rain echoes. The expected performances are studied analytically and compared with those of the approach of W. Hitschfeld and J. Bordan (1954). The stability and accuracy of the rainfall profile retrieval in the presence of various types of errors are pointed out and illustrated by means of numerical simulations of data processing from a spaceborne rain radar operating basically at Ku band     

Effect of multiple scattering on the estimation of rainfall ratesusing dual-wavelength radar techniques

The effect of multiple scattering on the estimation of rainfall rates using incoherent backscattered power at two frequencies (16 and 34.8 GHz) is analyzed. Simulation of circularly polarized transmissions propagating through homogeneous rain is used to estimate the rainfall rates from echo power levels calculated with and without multiple scattering effects included. The results indicate that in homogeneous rain, the rainfall rate estimated using the multiple scattering calculation is always smaller than that using the conventional radar equation. The difference is significant for heavy rain cases. Since the multiple scattering process may exist in real rain situations, one must be careful in heavy rain cases when interpreting rainfall rates estimated using dual-wavelength algorithms     

Monitoring of rain water storage in forests with satellite radar

The sensitivity of radar backscatter to the amount of intercepted rain in temperate deciduous forests is analyzed to determine the feasibility of retrieval of this parameter from satellite radar data. A backscatter model is validated with X-band radar measurements of a single tree exposed to rain. A good agreement between simulation and measurements is observed and this demonstrates the ability of radar to measure the amount of intercepted rain. The backscatter model is next applied to simulate different satellite radar configurations. To account for forest variability, the backscatter difference between a wet and dry forest canopy is calculated for four deciduous tree species, above a wet and a dry soil. On average, the simulated backscatter of a wet forest canopy is 1 dB higher than the backscatter of a dry forest canopy at co-polarized L-band and 2 dB at co-polarized C and X-band. The simulated sensitivity is in agreement with observations. It is argued that current satellites can retrieve the amount of intercepted rain at best with a reliability of 50%, due to the variability in soil moisture, species composition and system noise. The authors expect that the reliability will improve with the launch of the next generation radar satellites. The results of this analysis may also be used to assess the influence of rain, fog or dew upon other radar applications for temperate deciduous forests     

Backward and forward scattering by the melting layer composed ofspheroidal hydrometeors at 5-100 GHz

This paper addresses the behavior of the differential reflectivity, specific attenuation, and specific phase shift due to a melting layer composed of oblate-spheroidal hydrometeors. The results are based on a melting layer model and scattering computations derived from the point-matching technique with the truncation and recurrence adjusted. Computations at 5-100 GHz for five raindrop size distributions at rain rates below 12.5 mm/h are presented. In general, the reflectivity factor and differential reflectivity features with height at centimeter wavelengths agree with available radar measurements. At millimeter wavelengths, contributions to the radar backscatter from smaller hydrometeors become more and more important as the frequency increases and approaches 100 GHz. This should be instructive for utilizing millimeter wavelength radar techniques in radar remote sensing studies of the melting layer. Corresponding vertical profiles of the specific attenuation and phase shift are also presented at 5-100 GHz. The differential attenuation and phase shift indicate the particle shape effects. These attenuation and phase shift become more and more considerable as the frequency increases. Such forward scattering calculations should prove useful for studying propagation effects caused by the melting layer for satellite-earth communications, including depolarizations     

Simultaneous wind and rain retrieval using SeaWinds data

The SeaWinds scatterometers onboard the QuikSCAT and the Advanced Earth Observing Satellite 2 measure ocean winds on a global scale via the relationship between the normalized radar backscattering cross section of the ocean and the vector wind. The current wind retrieval method ignores scattering and attenuation of ocean rain, which alter backscatter measurements and corrupt retrieved winds. Using a simple rain backscatter and attenuation model, two methods of improving wind estimation in the presence of rain are evaluated. First, if no suitable prior knowledge of the rain rate is available, a maximum-likelihood estimation technique is used to simultaneously retrieve the wind velocity and rain rate. Second, when a suitable outside estimate of the rain rate is available, wind retrieval is performed by correcting the wind geophysical model function for the known rain via the rain backscatter model. The new retrieval techniques are evaluated via simulation and validation with data from the National Centers for Environmental Prediction and the Tropical Rainfall Measuring Mission Precipitation Radar. The simultaneous wind/rain estimation method yields most accurate winds in the "sweet spot" of SeaWinds' swath. On the outer-beam edges of the swath, simultaneous wind/rain estimation is not usable. Wind speeds from simultaneous wind/rain retrieval are nearly unbiased for all rain rates and wind speeds, while conventionally retrieved wind speeds become increasingly biased with rain rate. A synoptic example demonstrates that the new method is capable of reducing the rain-induced wind vector error while producing a consistent (yet noisy) estimate of the rain rate.     

Backscatter and attenuation by falling snow and rain at 96, 140,and 225 GHz

The results of measurements are presented for backscatter cross section per unit volume and attenuation for falling snow and rain at 96, 140, and 225 GHz. The attenuation due to rain is almost independent of the measurement frequency, but for snow the attenuation is considerably greater at 225 GHz than at 96 GHz. The rain attenuation generally varies with the rain accumulation rate in accordance with an aR^b relationship for a Laws and Parsons drop-size distribution where R is the rain rate and a and b are constants. The attenuation at all three frequencies is about 3 dB/km for a rain rate of 4 mm/h. The attenuation due to snow varies with airborne snow-mass concentration, with the average rates of increase being 0.9, 2.5, and 8.7 (dB/km)(g/m³) at 96, 140, and 225 GHz, respectively. Generally the attenuation for snow is lower than that for rain. The backscatter cross section per unit volume for rain at 96 GHz is about -35 dB m²/m³ for a rain rate of 4 mm/h. The backscatter from snow at 96 GHz is much lower than that from rain under equivalent accumulation rates or airborne mass concentrations. Snow backscatter at 140 GHz is comparable but higher than that at 96 GHz     

An Artificial-Neural-Network-Based Integrated Regional Model for Rain Retrieval Over Land and Ocean

An integrated regional model is proposed for rain-rate retrievals over land/ocean from the brightness temperature (Tb) values of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). The polarization-corrected temperature calculated from the 85.5-GHz channels is also considered as one of the inputs along with the nine channel Tb values. This model is applicable over the region between and . For this purpose, an artificial neural network is utilized. The collocated precipitation radar (PR) near-surface rain rates as given by a 2A25 data product is considered as a target value. The methodology consists of the separation of land and ocean pixels, the separation of stratiform and convective pixels over land/ocean, and the selection of important features (inputs) for the multilayer perceptron network by the feature selection technique for each group. For the separation of land/ocean pixels, the Tb values of the 10.65-GHz vertical channel are utilized. The values are utilized to separate the stratiform and convective pixels both over land and ocean. The rain retrieval from the developed model is validated with TRMM PR. Overall result shows the better agreement of the model-retrieved rain rate with the PR observation compared to the TMI (2A12) rain rate particularly over land. The rain retrieved from the developed model is further validated with Doppler weather radar. A reasonably good agreement is observed between these two estimations.     

Development of rain-attenuation and rain-rate maps for satellite system design in the Ku and Ka bands in Colombia

The predictions of rain rate and rain attenuation are the most important steps when analyzing a satellite link operating in the Ku and Ka bands. It can be a time-consuming process, especially when the analyses are made on a large number of sites, as might be the case with the broadband satellite systems of today. In this work, tools for the prediction of rain rate and rain attenuation are given in the form of contour maps for a tropical region (Colombia). The maps presented use the rain rate predicted by different methodologies, and the attenuation caused by these rain rates, using ITU recommendations. The information from these maps can then be entered into attenuation-prediction or system-planning tools. Additionally, a review of the results of the most important rain-rate and rain-attenuation campaigns is presented.     

Rain backscatter measurements at millimeter wavelengths

The US Army Ballistic Research Laboratory (BRL) conducted an experiment in 1973 to measure the properties of radar backscatter from rain at millimeter wavelengths. Rain backscatter and attenuation were measured with pulse radars operating simultaneously at 9.375, 35, 70, and 95 GHz over a wide range of rain intensities while continuous measurements of raindrop size and rainfall rate were made. This report describes the measurement technique, details of the instrumentation, the data analysis procedure, and the rain backscatter data obtained from A-scope photographs and video tapes     

TRMM卫星测雨雷达的数据处理及台风三维数据分析

研究星载测雨雷达的数据处理方法对于理解测雨雷达系统、开展数据应用以及研制星载气象雷达、开发反演算法等具有重要的意义。该文使用星载测雨雷达的真实数据计算了雷达反射率因子,以较小的计算量获得了准确的表面雷达截面积和路径积分衰减,并研究了降雨分类;得到了台风三维数据处理的图像结果。所得到的处理结果可为理解降水的内部结构与降水机制提供重要信息。     

Scattering of radiowaves by a melting layer of precipitation inbackward and forward directions

A melting layer of precipitation is composed of melting snowflakes (snow particles); the assumption of spherical particles along with mass conservation is used. The melting layer is studied by deriving the size distribution of the melting snow particles, the thickness of a melting layer, the density of a dry snow particle, and the average dielectric constant of a melting snow particle. Vertical profiles of radar reflectivity and specific attenuation are computed at 1-100 GHz by using the Mie theory for five raindrop size distributions at rain rates below 12.5 mm/h. The radar bright band is explained with computed radar reflectivities at 3-10 GHz. It is shown that the radar bright band can be absent in the melting layer at frequencies above 20 GHz. This agrees with radar observations at 35 and 94 GHz. The specific attenuation, as well as the average specific attenuation of the melting layer, is divided into absorption part and scattering part. The latter is increasingly significant with the increase of frequency. The total zenith attenuation due to stratiform rain is divided into the rain zenith attenuation and the additional zenith attenuation, which is the difference between zenith attenuation, due to the melting layer, and attenuation, due to the same path length of the resulting rain. The additional zenith attenuation increases with the increase of rain rate even at frequencies above 20 GHz. This should be taken into account in radar remote sensing and satellite-Earth communications     

Useful experimental results for Earth-satellite rain attenuation modeling

Results pertaining to vertical profiles of rain rates derived from radar data as well as normalized cumulative probabilities of rain rates, which may be applicable over large areas of the world, are presented. We believe these results to be useful in the areas of earth-satellite rain attenuation modeling and prediction at frequencies above 10 GHz.