Detection of Ice Hydrometeor Alignment Using an Airborne W-band Polarimetric Radar

This paper presents airborne W-band polarimetric radar measurements at horizontal and vertical incidence on ice clouds using a 95-GHz radar on the University of Wyoming King Air research aircraft. Coincident, in situ measurements from probes on the King Air make it possible to interpret polarimetric results in terms of hydrometeor composition, phase, and orientation. One of the key polarimetric measurements recently added to those possible with the W-band radar data system is the copolar correlation coefficient rHV. A discussion of the relation between cloud scattering properties and rHV covers a test for isotropy of the distribution of observed hydrometeors in the plane of polarization and qualitative evaluation of the possible impact of Mie (resonant) scattering on rHV measurements made at W band. Prior measurements of rHV at S band and Ku band are compared with the W-band results. The technique used to measure rHV, including the real-time and postprocessing steps required, is explained, with a discussion of the expected measurement error for the magnitude and phase of rHV. Cloud data presented include melting-layer observations at vertical incidence, observation of a convective snow cell at vertical incidence, and observations of needle crystals at both horizontal and vertical incidence. The melting layer observations provide a consistency check for the measurements of rHV and linear depolarization ratio (LDR) at W band through the test for isotropy. The vertical incidence measurements of a convective snow cell displayed significant mean orientation of the hydrometeors observed in the features evident in ZDR and the phase of rHV. Data taken on needle crystals provided clear indication of particle alignment in the measurements of ZDR and LDR for the horizontal incidence case and equally clear indication of a lack of orientation for the vertical incidence case.