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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 HV. A discussion of the relation between cloud scattering properties and HV 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 HV measurements made at W band. Prior measurements of HV at S band and Ku band are compared with the W-band results. The technique used to measure HV , including the real-time and postprocessing steps required, is explained, with a discussion of the expected measurement error for the magnitude and phase of HV. 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 HV 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 Z DR and the phase of HV. Data taken on needle crystals provided clear indication of particle alignment in the measurements of Z DR and LDR for the horizontal incidence case and equally clear indication of a lack of orientation for the vertical incidence case.

An experiment whose objective was to determine the wind and reflectivity substructure of severe convective storms is detailed. A 3-mm-wavelength (95 GHz) pulsed Doppler radar was installed in a van and operated in the Southern Plains of the United States during May and early June of 1993 and 1994. Using a narrow-beam antenna with computer-controlled scanning and positioning the van several kilometers from targets in severe thunderstorms, the authors were able to achieve 30-m spatial resolution and also obtain video documentation. A dual-polarization pulse-pair technique was used to realize a maximum unambiguous velocity of ±80 m s _1. Analyses of data collected in a mesocyclone near the intersection of two squall lines, in a low-precipitation storm, and in a hook echo in a supercell are discussed. A strategy to achieve 10-m spatial resolution and obtain analyses of the internal structure of tornadoes is proposed.

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Investigation of precipitation formation requires measurements of the drop size distribution in a cloud. These measurements have usually been made using ground-based radar systems or aircraft in situ probes. Difficulties encountered in practice using these systems include accounting for the air motion at points remote from the radar systems and small sample volumes measured using the aircraft probes. An airborne W-band radar system provides a measurement from a much larger sample volume, close to the aircraft, with a correction for air motion possible using the data from the aircraft inertial navigation system. The Coastal Stratus Experiment conducted off the coast of Oregon in late 1995 provided W-band radar and microphysical probe data sampled from much of the same region of a marine stratus cloud. The unique combination of cloud probes and W-band radar on board the University of Wyoming King Air allowed the radar sampling to be only 60 m away from the probe sampling region. Doppler spectrum data from the W-band radar were used to produce estimates of the drop size spectrum density N(D). These estimates were compared to measurements of N(D) taken by the Particle Measuring Systems forward scattering spectrometer, 1D, and 2DC probes. This comparison suggests that a vertically pointing airborne W-band radar is a viable remote sensing tool for measuring N(D) in clouds and precipitation. This radar provides information on drop size distribution variation on a much smaller horizontal scale than the probes as a result of the much higher sample rate and larger measurement sample volume.