This paper reviews the theories and the results of numerical calculations of interactions between hydrometeors and microwaves or millimeter waves propagating in media containing these hydrometeors. At frequencies above about 5 GHz, rain, snow, or ice crystals in high altitudes cause various degrading effects on the performance of communication links. The attenuation of the transmitted signals is a well-known example of such effects. Because of the demand for more communication channels, operating frequencies of both terrestrial and earth--scace communication systems become increasingly higher, and the communication technology becomes more and more sophisticated with the use of digital techniques, or the use of both orthogonaly polarized channels. These have given rise to the need of acquiring more detailed knowledge of interactions between hydrometeors and waves than is necessary for the calculation of attenuation. To obtain such knowledge, we have to know the detailed microphysical properties of hydrometeors, such as shape, size distributions, or the motion of hydrometeors while they are falling. These properties, some obtained previously by meteorolgists and some obtaianed recently by radio engineers to meet the requirements of present radio problems, are reviewed first. Various methods of calculations, and the detailed knowledge obtained from calculations of the single scattering properties of hydrometeors, whose sizes are generally comparable to wavelength, are then reviewed. With the information of these scattering properties, various propagation phenomena are discussed from a theoretical point of view. Specific subjects treated include attenuation, cross polarization due to rain or ice crystals in high altitudes, the methods of depolarization cancellation, rain scattering other then forward and backward which is necessary in the estimation of rain scatter interference between two communication links operating at the same frequency, various multiple or incoherent scattering effects on copolar and cross-polar signals, pulse shape, channel transfer characteristics, etc. The aim of this review is to provide fundamental background informations on the interactions between hydrometeors and radio waves, and hence the more practical investigations, such as long-term statistics of attenuation, are beyond the scope of this review.

Electromagnetic wave propagation and scattering in rain and other hydrometeors

Drop shapes derived from a previously conducted artificial rain experiment using a two-dimensional video disdrometer (2DVD) are presented. The experiment involved drops falling over a distance of 80 m to achieve their terminal velocities as well as steady-state oscillations. The previous study analyzed the measured axis ratios (i.e., ratio of maximum vertical to maximum horizontal chord) as a function of equivolumetric spherical drop diameter (Deq) for over 115 000 drops ranging from 1.5 to 9 mm. In this paper, the actual contoured shapes of the drops are reported, taking into account the finite quantization limits of the instrument. The shapes were derived from the fast line-scanning cameras of the 2DVD. The drops were categorized into Deq intervals of 0.25-mm width and the smoothed contours for each drop category were superimposed on each other to obtain their most probable shapes and their variations due to drop oscillations. The most probable shapes show deviation from oblate spheroids for De...

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Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain

Both scattering and turbulence can effect the spatial coherence of short wavelength signals propagating through the open atmosphere. In this paper, the influence of forward scattering on heterodyne receiver performance is investigated, taking into account turbulence. It is shown that the effect of forward scattering is to reduce the effective heterodyne receiver area through spatial coherence degradation. A common approach to scattering as an attenuation phenomenon is not always valid. Generally, this approach underestimates the SNR. The accuracy of the attenuation approach depends on the ratio R of the actual receiver diameter to the scattering particle diameter. If R > 100, scattering is essentially large angle and the typical treatment of scattering as an attenuation effect is indeed justified. However, for small R, forward scattering is primarily small angle, field coherence is noticeably affected by forward scattering, and the attenuation approach is not valid. Further, it is shown that the SNR is improved when the ratio of the scattering particulate size to turbulence coherence diameter decreases. From the practical point of view, the most important result of this study is that small receivers use their area more effectively than large receivers. Thus, an array of several small receivers may perform better than one large receiver with the same total area. The treatment here is particularly relevant for coherent detection through clouds, fog, precipitation, and turbid media in general, including liquid media.

Heterodyne detection through rain, snow, and turbid media: effective receiver size at optical through millimeter wavelengths.

Investigation of the scattering properties of hydrometeors, especially those of nonspherical hydrometeors, has become increasingly important in connection with the estimation of depolarization due to hydrometeors in terrestrial and earth-space microwave communication systems. Scattering from nonspherical hydrometeors is also important in remote sensing of precipitation parameters. This paper first reviews various analytical-numerical approaches for the calculation of single-scattering properties of nonspherical hydrometeors. Approaches for the analysis of the practical problems, such as depolarization phenomena and rain scatter interference, are outlined in order to see how the single-scattering calculations are related to these problems. The present knowledge of incoherent scattering effects on rain attenuation, cross polarization, and channel transfer characteristics at the microwave and millimeter wave region is also reviewed.

Scattering from hydrometeors: A survey

A simple model of the radar scattering by atmospheric particles is used to interpret all elements of the covariance scattering matrix. The components of the covariance scattering matrix and corresponding polarimetric variables are expressed via a limited number of integral parameters that characterize distributions of sizes, shapes, and orientations of meteorological scatterers. The co–cross-polar correlation coefficients ρxh and ρxυ measured in the horizontal–vertical linear polarization basis are the major focus of this study. It is shown that the magnitudes of both coefficients are almost entirely determined by orientation of particles and do not depend on particle sizes and shapes. The phases of these coefficients can be used to detect the presence of melting hail or wet snow in the radar resolution volume. A model of the mean canting angle of raindrops varying along a propagation path is developed to examine effects of propagation on the depolarization variables such as ρxh, ρxυ, and linear ...

Interpretation of Polarimetric Radar Covariance Matrix for Meteorological Scatterers: Theoretical Analysis

This paper reviews recent theoretical and experimental research on cross polarization that occurs during precipitation on terrestrial microwave links. Consideration is first given to the physical mechanisms, such as raindrop canting, that cause precipitation depolarization. Coherent rain depolarization models are then discussed and recent investigations of incoherent depolarization effects summarized. Particular attention is given to semi-empirical models since they can be used most readily for statistical predictions in design applications. ‘Model oriented’ cross polarization measurements aimed at characterizing the physical properties of the depolarizing medium for use in model development are also given particular attention. Effective parameters of the raindrop canting angle distribution derived from these measurements are tabulated. Finally, some mention is made of the relative importance of rain depolarization and attenuation in causing outages on digital radio systems.

R. L. Olsen

Papers

Cross polarization during precipitation on terrestrial links: A review