Single scattering from frozen hydrometeors for cloud radar applications

In order to observe solid precipitation by ground-based as well as space-borne cloud radars, precise knowledge of the scattering behavior of frozen hydrometeors at centimeter- and millimeter-wavelengths is required. Only for homogeneous spherical particles, the scattering of electromagnetic waves can be computed in a mathematically exact way using the conventional Mie-theory. However, frozen hydrometeors exist in a huge variety of nonspherical shapes. In this study single scattering parameters (radar cross section, scattering cross section, absorption cross section) for simplified shape classes of frozen hydrometeors were calculated using the Discrete Dipole Approximation (DDA) method. The particles were modeled as hexagonal plates, columns, needles and dendrites as well as rather spherical graupel particles. Also arbitrarily shaped particles were modeled from their contour images, recorded by a 2D-video-distrometer. The calculations were carried out over a wide range of centimeter- and millimeter-wavelengths, from 1 to 100 GHz. The study results show that for arbitrary frozen hydrometeors smaller than 1/10 of the wavelength, exclusively their ice volume is crucial for scattering and absorption. For such small sizes the actual shape of the particle needs not to be modeled - just the Mie solution of an equivolumetric ice sphere can be used to calculate scattering parameters. The results also indicate that for nearly spherical particles like graupel consisting of a mixture of ice and air, equivolumetric ice spheres can be used as approximations even up to particle diameters of a half wavelength.