Evaluation of a 45° Slant Quasi-Linear Radar Polarization State for Distinguishing Drizzle Droplets, Pristine Ice Crystals, and Less Regular Ice Particles

Abstract

A remote sensing capability is needed to detect clouds of supercooled, drizzle-sized droplets, which are a major aircraft icing hazard. Discrimination among clouds of differing ice particle types is also important because both the presence and type of ice influence the survival of liquid in a cloud and the chances for occurrence of these large, most hazardous droplets. This work shows how millimeter-wavelength dual-polarization radar can be used to identify these differing hydrometeors. It also shows that by measuring the depolarization ratio (DR), the estimation of the hydrometeor type can be accomplished deterministically for drizzle droplets; ice particles of regular shapes; and to a considerable extent, the more irregular ice particles, and that discrimination is strongly influenced by the polarization state of the transmitted microwave radiation. Thus, appropriate selection of the polarization state is emphasized. The selection of an optimal polarization state involves trade-offs in competing factors such as the functional dynamic range of DR, sensitivity to low-reflectivity clouds, and insensitivity to oscillations in the settling orientations of ice crystals. A 45° slant, quasi-linear polarization state, one in which only slight ellipticity is introduced, was found to offer a very good compromise, providing considerable advantages over standard horizontal and substantially elliptical polarizations. This was determined by theoretical scattering calculations that were verified experimentally in field measurements conducted during the Mount Washington Icing Sensors Project (MWISP). A selectable-dual-polarization Ka-band (8.66-mm wavelength) radar was used. A wide variety of hydrometeor types was sampled. Clear differentiation among planar crystals, columnar crystals, and drizzle droplets was achieved. Also, differentiation among crystals of fundamentally different shapes (aspect ratios) within each of the planar and columnar families was found possible. These distinctions matched calculations of DR, usually to within 1 or 2 dB. The results from MWISP and from previous experiments with other polarizations have demonstrated that the agreement between theory and measurements by this method is repeatable. Additionally, although less rigorously predicted by theory, the field measurements demonstrated substantial differentiation among the more irregular and more spherical ice particles, including aggregates, elongated aggregates, heavily rimed dendrites, and graupel. Measurable separation between these various irregular ice particle types and drizzle droplets was also verified.