An approach is suggested to relate measurements of radar depolarization ratios and aspect ratios of predominant hydrometeors in nonprecipitating and weakly precipitating layers of winter clouds. The trends of elevation angle dependencies of depolarization ratios are first used to distinguish between columnar-type and plate-type particles. For the established particle type, values of depolarization ratios observed at certain elevation angles, for which the influence of particle orientation is minimal, are then used to estimate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polarizations, including circular, slant-45Њ linear, and two elliptical polarizations, is discussed. These two elliptical polarizations are quasi-circular and quasi-linear slant-45Њ linear, and both are currently achievable with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory's K a-band radar. In comparison with the true circular and slant-45Њ linear polarizations, the discussed elliptical polarizations provide a stronger signal in the ''weak'' radar receiver channel; however, it is at the expense of diminished dynamic range of depolarization ratio variations. For depolarization measurements at the radar elevation angles that do not show much sensitivity to particle orientations, the available quasi-circular polarization provides a better depolarization contrast between nonspherical and spherical particles than does the available quasi-linear slant-45Њpolarization. The use of the proposed approach is illustrated with the experimental data collected during a recent field experiment. It is shown that it allows successful differentiation among pristine planar crystals, rimed planar crystals, long columns, blocky columns, and graupel. When a reasonable assumption about particle bulk density is made, quantitative estimates of particle aspect ratios from radar depolarization data are in good agreement with in situ observations. Uncertainties of particle aspect ratios estimated from depolarization measurements due to 0.1 g cm Ϫ3 variations in the assumed bulk density are about 0.1.

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (K DP and ␦ o , respectively) in rain are discussed for X-(ϳ 3 cm) and K a-band (ϳ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K DP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h Ϫ1 is generally small and should not pose a serious problem when estimating K DP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (ϳ 10-11 cm) wavelengths is an increase in K DP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at K a band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from K a-band measurements of the differential phase. Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4-5 mm h Ϫ1. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A K DP-R relation derived for the mean drop aspect ratio (R ϭ 20.5) provided a satisfactory agreement between rain accumulations derived from radar 0.80 K DP measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

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.

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