IDENTIFICATION OF HYDROMETEORS WITH ELLIPTIC AND LINEAR-POLARIZATION K-A-BAND RADAR

Polarimetric radar can be used to identify various types of hydrometeo rs. Ice crystals of the varied growth habits depolarize and backscatte r millimeter-wavelength radiation according to crystal aspect ratio, b ulk density, and orientation, and the polarization state of the incide nt radiation. In this paper model calculations of the depolarization c aused by various crystal types are extended from previous work, and K- a-band (8.66 mm) radar measurements of linear and elliptical depolariz ation ratios (LDR and EDR) from various ice hydrometeors are presented . The measurements for regular crystals are related to the models. Dri zzle drops, which are quasi-spherical, serve as a reference. Signature discrimination in cloud systems with more than one type of hydrometeo r is addressed. The model calculations illustrate the interplay of the parameters that control depolarization. They predict that in the depo larization signatures, crystals of the various basic planar and column ar habits should generally be most separable, one habit group from ano ther and, to a degree, within each group when they occur in common, ma ture size distributions. It is verified in this and related papers tha t measurements of depolarization with a K-a-band dual-polarization rad ar provide good estimates of hydrometeor identity to separately distin guish drizzle, pristine crystals of various growth habits, graupel, an d aggregates in winter storm clouds that have reasonable horizontal ho mogeneity over short distances (similar to 10-20 km). Characterization of the mix of two or three hydrometeor types is also possible, once t he individual types are identified in some part of the cloud. Quantita tive agreement between the measurements and the models, supported by s now crystal samples, was much better for EDR than for LDR; that is, ED R enabled more specific hydrometeor identification. However, LDR provi ded indications of randomness of crystal orientation and a wider decib el gap differentiating graupel from drizzle.