Identification of Hydrometeors with Elliptical and Linear Polarization Ka-Band Radar

Abstract

Polarimetric radar can be used to identify various types of hydrometeors. Ice crystals of the varied growth habits depolarize and backscatter millimeter-wavelength radiation according to crystal aspect ratio, bulk density, and orientation, and the polarization state of the incident radiation. In this paper model calculations of the depolarization caused by various crystal types are extended from previous work, and Ka-band (8.66 mm) radar measurements of linear and elliptical depolarization ratios (LDR and EDR) from various ice hydrometeors are presented. The measurements for regular crystals are related to the models. Drizzle drops, which are quasi-spherical, serve as a reference. Signature discrimination in cloud systems with more than one type of hydrometeor is addressed. The model calculations illustrate the interplay of the parameters that control depolarization. They predict that in the depolarization signatures, crystals of the various basic planar and columnar habits should generally be most separable, one habit group from another and, to a degree, within each group when they occur in common, mature size distributions. It is verified in this and related papers that measurements of depolarization with a Ka-band dual-polarization radar provide good estimates of hydrometeor identity to separately distinguish drizzle, pristine crystals of various growth habits, graupel, and aggregates in winter storm clouds that have reasonable horizontal homogeneity over short distances (?10?20 km). Characterization of the mix of two or three hydrometeor types is also possible, once the individual types are identified in some part of the cloud. Quantitative agreement between the measurements and the models, supported by snow crystal samples, was much better for EDR than for LDR; that is, EDR enabled more specific hydrometeor identification. However, LDR provided indications of randomness of crystal orientation and a wider decibel gap differentiating graupel from drizzle.