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     

Predictions of radiowave attenuations due to a melting layer ofprecipitation

A melting layer model related to the physical constants and meteorological parameters is employed in this investigation. The specific phase shift, together with the specific attenuation, is computed at 1-100 GHz by using the Mie theory. The additional zenith attenuation, which is the difference between zenith attenuation due to the melting layer and attenuation due to the same thickness of the resulting rain, is comprehensively studied. The ratio of the difference to rain zenith attenuation may be over 1 at 1-5 GHz although the difference is much less than 1 dB. The difference can be over 1 dB at frequencies above 20 GHz. A minimum of the ratio is below 0.05 at frequencies about 40-60 GHz but the ratio can become a value of about 0.1 at 100 GHz. The additional attenuation should be taken into account in satellite-Earth communications and radar remote sensing. The power law parameters of the average specific attenuation of the melting layer and rain specific attenuation are tabulated for three raindrop size distributions at rain rates of below 25 mm/h. The power law method could be utilized in the additional attenuation calculation. It is a good approximation of the Mie theory results at 1-50 GHz and a useful estimate at 50-100 GHz     

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.     

A New Melting Particle Model and its Application to Scattering of Radiowaves by a Melting Layer of Precipitation

A melting layer of precipitation is composed of melting snow particles, and we modeled them by three-layered spherical particles, in which the innermost layer is air, the middle ice and the outermost water. Based on this model, the radar reflectivity, together with the specific phase shift and the specific attenuation of a melting layer of precipitation, were computed at 1–100 GHZ by using the Mie theory. The radar bright band is explained by this model. We compared our numerical results with that in the literature Zhang (IEEE Transactions on Antennas and Propagation 42(3):347–356, 1994), Zhang (IEEE Transactions on Antennas and Propagation 42(3):492–500, 1994). It demonstrates that the three-layered snow sphere model is appropriate and practicable, so the computed results are more accurate. This study can be used in radar remote sensing and satellite-earth communications.     

Melting layer attenuation at 28.6 GHz from simultaneous comstar beacon and polarisation diversity radar data

Attenuation data at 28.6 GHz obtained from measurements of the Comstar beacon show that, for moderate rain, slant path attenuation may significantly exceed that calculated from simultaneous radar reflectivity measurements. Polarisation diversity radar data were used for positive identification of the rain and the melting layer, and for estimating the rain attenuation along the path. These results indicate that the melting layer attenuation is significant.     

A simplified approach to the evaluation of EMW propagationcharacteristics in rain and melting snow

Electromagnetic propagation characteristics in rain and melting snow are calculated by treating these media as artificial dielectrics. Computed values of attenuation and phase shift in rain, obtained by this approach, are compared with those derived by using Mie scattering theory over a frequency range of 1-1000 GHz and for rain rates up to 100 mm/h. Very close agreement is generally obtained over these entire ranges. Melting snow is treated in the same manner, where comparison is possible these results tend to agree well with the available, but rather limited, published data. Attenuation and phase shift are calculated as a function of the degree of melting of the snow particles. Subject to assumptions relating the degree of melting to depth in the melting layer, average values of attenuation and phase shift are computed as functions of frequency. The attenuation values compare well with those derived from an empirical formula over the range of its validity     

Airborne active and passive microwave observations of Super TyphoonFlo

Airborne microwave measurements of precipitation associated with Super Typhoon Flo in the western North Pacific were conducted during September 16-18, 1990. The sensor package aboard the NASA DC-8 aircraft included a dual-frequency precipitation radar at 10 GHz and 34 GHz and a host of radiometers operating at 10 GHz, 18 GHz, 19 GHz, 34 GHz, and 92 GHz, as well as three frequencies near the strong water vapor absorption line of 183.3 GHz. The measurements were made during a few passes over the storm center, and active and passive microwave signatures of the rainbands were detected with a fine spatial resolution. The relationship between the measured brightness temperature and radar-estimated rain rate is examined at the frequencies between 10-92 GHz. At both 34 and 92 GHz this relationship is analyzed with the 10 GHz radar reflectivity factor measured at altitudes above the freezing layer as a further constraint. The results show that frozen hydrometeors strongly scatter radiation at these frequencies, especially at 92 GHz. It was shown from a close examination of both active and passive microwave signatures that a significant scattering of radiation at frequencies 118 GHz occurred in the inner eyewall at altitudes of 3-8 km. This scattering of microwave radiation by hydrometeors in both liquid and frozen forms is discussed under the authors' current understanding of the scattering mechanism     

Attenuation of co- and cross-polarized electric fields of wavesthrough a layer of dielectric spheroids

A recent model (see D.A. de Wolf et al., ibid., vol.38, p.1317-25, Sept. 1990) for analyzing radar reflections from the layer of melting ice hydrometeors is adjusted and expanded for analyzing line-of-sight attenuation of copolar and crosspolar signals at wavelengths that are comparable to the particle sizes. Expressions for copolar and cross-polar attenuation discrimination are derived. It is shown with a parametric study that these quantities have the expected behavior     

Ice-crystal depolarisation measurements at 28.6 GHz with simultaneous 16.5 GHz polarisation diversity radar observations

Simultaneous measurements with a 16.5 GHz polarisation diversity radar and the 28.56 GHz Comstar beacon are reported. Radar measurements of the differential propagation constant during a severe depolarisation event established that the depolarisation was due primarily to lossless scatterers above the melting layer, with little contribution from rain or the melting layer. Differential phase shifts as high as 1.7°/km at 16.5 GHz, corresponding to 2.9°/km at 28.56 GHz, were observed. Good agreement between radar derived and beacon measurements of differential phase shift was obtained.     

The effects of rain clutter on millimeter radar performance

The effects of rain clutter on millimeter radar performance are investigated at 35, 94 and 140GHz frequencies, including rain attenuation, radar reflectivity, maximum radar range and equivalent target cross section.     

The Chilbolton advanced meteorological radar: a tool formultidisciplinary atmospheric research

An ex-air-surveillance S-band radar was installed on the 25 m antenna at Chilbolton, near Winchester, over 20 years ago for studies of the effects of rain on communications systems. Since that time the functionality of the radar has been steadily developed to keep it at the forefront of meteorological research radars throughout the world. Recent developments have added phase and Doppler capabilities to the radar. The paper briefly discusses the use of differential reflectivity measurements in determining rainfall rates, microwave attenuation and the phase of hydrometeors. The new hardware and signal processing are described and examples of radar scans and applications of the radar are presented     

Attenuation in melting snow on microwave- and millimetre-wave terrestrial radio links

The scattering properties of melting snow on microwave and millimetre-wave terrestrial radio links are predicted using a new model for melting which includes coalescence. Attenuation, differential attenuation and differential phase are calculated for a horizontal path, with results at 36.25 GHz presented. Peak specific attenuation in the range 8?13 dB/km is expected for underspread rain with 10?15 mm/h rain rates.     

Calculations of millimeter wave depolarization due to melting layer and rain

A model for calculating the total depolarization due to the melting layer and rain is proposed under the assumption that oblate spheroidal melting particles and raindrops have the same orientation. The melting layer is composed of the melting particles which are made up from the mixture of ice, air and water. The specific attenuation and the specific phase shift both for the melting layer and for rain are given in the power lawaR ^b form for the rain rates 0≤R≤12.5 mm/h and the parameters are tabled over the frequency range of 1–100 GHz. Using the model, the numerical calculation of the depolarization is possible for three drop size distributions.     

Effects of drop size and temperature on forward and backward scattering of microwaves by raindrops

An explanation is given for the peak sensitivity around 9?11 GHz to drop-size distribution and temperature of microwave attenuation due to a rain-filled medium. The sensitivity of radar reflectivity, differential radar reflectivity and differential radar cross-section to drop-size and temperature are also given. It is shown that at frequencies in excess of about 25 GHz the vertical radar cross-section of water drops can be greater than the horizontal.     

Characteristics of rain induced attenuation and phase shift at cm and mm waves using a tropical raindrop size distribution model

The tropical raindrop size distribution model developed by Ajayi and Olsen has been employed to study some characteristics of rain induced attenuation and phase shift for a tropical location for spherical, oblate spheroidal and Pruppacher-Pitter drop shapes. Parameters such as the a and b values for the power law relation between the specific attenuation and rainfall rate as well as differential attenuation and phase shift and their normalized values, were computed. A single power law between the specific phase shift and the rain rate was found to be adequate for vertical polarization, whilst a two-segment power law fitting is required for horizontal polarization between 1GHz and about 100GHz. The results were compared in many cases with those obtained with the Laws and Parsons drop size distribution, currently adopted by the CCIR for scattering applications.     

水基附着介质罩对毫米波衰减的理论分析与实验研究

本文主要研究多种可能出现的天线罩水基附着介质对毫米波所产生的衰减.本文采用一个四层介质模型来模拟水附着在天线罩上的情况,根据Fresnel方程得到150 GHz和298 GHz电磁波衰减模型,通过测量纯水对毫米波所产生的衰减验证模型的正确性;再测量雨水、海水所产生的衰减,分析常见水基附着介质对毫米波所产生的衰减;最后测量不同盐度的海水对毫米波所产生的衰减,分析了海水盐度对衰减的影响.实验结果证明毫米波的衰减随着天线罩上附着水层厚度的增加呈现出较为规律性增长,而此衰减受中水中溶解物（微小颗粒和盐度）的影响较小.本文对毫米波雷达在雨天等复杂室外条件下的应用具有一定的指导作用.     

A physical model to determine snowfall over land by microwave radiometry

Falling snow is an important component of global precipitation in extratropical regions. This paper describes the methodology and results of physically based retrievals of snow falling over land surfaces. Because microwave brightness temperatures emitted by snow-covered surfaces are highly variable, precipitating snow above such surfaces is difficult to observe using window channels that occur at low frequencies (/spl nu/<100 GHz). Furthermore, at frequencies /spl nu//spl les/37 GHz, sensitivity to liquid hydrometeors is dominant. These problems are mitigated at high frequencies (/spl nu/>100 GHz) where water vapor screens the surface emission, and sensitivity to frozen hydrometeors is significant. However, the scattering effect of snowfall in the atmosphere at those higher frequencies is also impacted by water vapor in the upper atmosphere. The theory of scattering by randomly oriented dry snow particles at high microwave frequencies appears to be better described by regarding snow as a concatenation of "equivalent" ice spheres rather than as a sphere with the effective dielectric constant of an air-ice mixture. An equivalent sphere snow scattering model was validated against high-frequency attenuation measurements. Satellite-based high-frequency observations from an Advanced Microwave Sounding Unit (AMSU-B) instrument during the March 5-6, 2001 New England blizzard were used to retrieve snowfall over land. Vertical distributions of snow, temperature, and relative humidity profiles were derived from the Mesoscale Model (MM5) cloud model. Those data were applied and modified in a radiative transfer model that derived brightness temperatures consistent with the AMSU-B observations. The retrieved snowfall distribution was validated with radar reflectivity measurements obtained from a ground-based radar network.     

基于EMD方法的X波段双偏振雷达衰减订正

针对X波段双偏振雷达信号在降雨路径中的衰减现象,本文提出经验模式分解（Empirical Mode Decomposition,EMD）方法进行X波段双偏振雷达衰减订正,首先对总差分传播相移进行EMD分解得到有限个基本模式分量（Intrinsic Mode Function,IMF）,并基于皮尔逊相关系数准则将IMF分为噪声IMF和信号IMF两类,然后对信号IMF进行有效重构得到差分传播相移,再将差分传播相移通过最小二乘法拟合得到差分传播相移率,最后对求得的差分传播相移与差分传播相移率采用自适应约束方法进行反射率衰减订正。利用EMD方法和其他方法进行对比分析,其结果表明,EMD方法能够有效地消除X波段双偏振雷达回波数据中后向散射的影响,在保留真实的气象信息的同时,有效地抑制差分传播相移的显著波动,进而衰减订正效果更好。     

Bistatic scattering of radiowaves by a melting layer ofprecipitation

This paper addresses bistatic scattering of radiowaves by the melting layer of precipitation. The bistatic radar reflectivities are formulated and can be computed at 1-100 GHz by applying the Mie theory for five raindrop-size distributions at rain rates below 12.5 mm/h. Examples computed at 35 GHz are presented. This original bistatic scatter calculation is not only of substantial guidance into bistatic scattering of radiowaves by the real melting layer of precipitation, but also should be appropriate for considering the interference problems including the melting layer effects for engineering purposes     

雾的雷达后向散射特性研究

利用流雾与辐射雾的能见度与含水量的经验公式导出的Gamma雾滴谱分布,得到了平流雾和辐射雾的雷达反射因子Z与雾的能见度、含水量的关系式。以94GHz雷达为例,研究了雾的雷达后向散射特性。最后利用毫米波衰减数据和能见度反演的几场湿海雾参数,研究了湿海雾的后向散射特性,研究结果表明：湿海雾的雷达反射因子可与中到大雨相当。