ISSN 0001-4338, Izvestiya, Atmospheric and Oceanic Physics, 2016, Vol. 52, No. 1, pp. 74–81. © Pleiades Publishing, Ltd., 2016. Original Russian Text © Ya.A. Ilyushin, B.G. Kutuza, 2016, published in Izvestiya AN. Fizika Atmosfery i Okeana, 2016, Vol. 52, No. 1, pp. 83–91. 74 1. INTRODUCTION Studying rainfall with the use of space observations is an important problem of remote sensing. The possi- bility to determine the rain rate from the data of obser- vations of microwave radiation was first demonstrated in 1968 by the results of the experiment onboard the Cosmos 243 Earth satellite [1]. Zones of precipitation over the sea surface were recorded from the character- istic values of brightness temperatures at wavelengths of 0.8, 1.35, and 3.2 cm. In this case, the precipitation rate can be determined by the radiation intensity. A further development of the technology for observing the atmospheric precipitation from space is associated with the US satellite DMSP having the microwave radiometer SSM/I [2] with an operating range of wavelengths from 0.35 to 1.6 cm. The precip- itation zones were determined as areas with a reduced brightness temperature at a wavelength of 0.35 cm because of the increase in the albedo with the multiple scattering of the microwaves on large raindrops. In recent years, a remote sensing of the atmospheric pre- cipitation has been conducted onboard the GPM and TRMM spacecrafts. With respect to these and other fields of research, the interaction of microwave radiation with precipi- tates and clouds of different types has been studied intensely [3, 4]. Many of them consist of particles of a nonspherical shape having a primary orientation (fall- ing raindrops, snow, and other crystalline particles of ice). Polarization effects play a substantial role in the propagation of radiation in these media. A combina- tion of the effects of scattering, the spatial inhomoge- neity of the precipitates in the atmosphere, and also, to some extent, the reflective properties of the surface leads to the necessity of considering fields of micro- wave radiation in a three-dimensionally inhomoge- neous medium with dichroism. At the present time, a relatively small number of works have been published regarding the numerical solving of the vector radiative transfer equation (VRTE) in the three-dimensionally inhomogeneous anisotropic scattering media [4]. This work is aimed at the theoret- ical estimation of the intensity and polarization of the thermal radio radiation of rainfall when they are observed by microwave radiometers from space. For achieving these aims, a number of problems, including the evaluation of the radiative properties of the medium (rain), construction of a numerical model of radiative transfer in a three-dimensional rain cell, and practical implementation of calculations using the created model, need to be solved. The last problem, due to the computational complexity of the calcula- tion, has not yet been solved and is the main part of the work presented here. 2. MODEL OF RADIATIVE CHARACTERISTICS OF THE MEDIUM Two models of rain atmosphere are studied and compared to each other: an isolated cell and a contin- uous flat layer of the medium that is homogeneously filled with falling raindrops. Following [4], we have chosen a model of a rain cell in the form of a cube of Influence of a Spatial Structure of Precipitates on Polarization Characteristics of the Outgoing Microwave Radiation of the Atmosphere Ya. A. Ilyushin a, b, c and B. G. Kutuza b a Department of Physics, Moscow State University, Moscow, Russia b Kotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 125009 Russia c All-Russia Research Institute of Optical and Physical Measurements, Moscow, 119361 Russia e-mail: ilyushin@physics.msu.ru Received February 4, 2014; in final form March 18, 2015 Abstract—Issues of the formation and recording of the spatial and angular distributions of thermal radio radia- tion of rainfall in the microwave range are discussed. The thermal radio-frequency radiative transfer in a three- dimensional rain cell is simulated numerically with a different rainfall rate, taking into account the nonspherical shape of falling raindrops and their size distribution. The role of the three-dimensional inhomogeneity of rainfall fields in the formation of a field of their inherent thermal radio radiation in the microwave range is revealed. Keywords: millimetric waves, rainfall, radiometer, radiative transfer DOI: 10.1134/S0001433816010047