On the use of radar depolarization ratios for estimating shapes of ice hydrometeors in winter clouds

An approach is suggested to relate measurements of radar depolarization rat ios and aspect ratios of predominant hydrometeors in nonprecipitating and w eakly 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 es timate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polariza tions, including circular, slant-45 degrees linear, and two elliptical pola rizations, is discussed. These two elliptical polarizations are quasi-circu lar and quasi-linear slant-45 degrees linear, and both are currently achiev able with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory's Ka-band radar. In comparison with the true circula r and slant-45 degrees linear polarizations, the discussed elliptical polar izations provide a stronger signal in the "weak'' radar receiver channel; h owever, it is at the expense of diminished dynamic range of depolarization ratio variations. For depolarization measurements at the radar elevation an gles that do not show much sensitivity to particle orientations, the availa ble quasi-circular polarization provides a better depolarization contrast b etween nonspherical and spherical particles than does the available quasi-l inear slant-45 degrees polarization. The use of the proposed approach is il lustrated with the experimental data collected during a recent field experi ment. It is shown that it allows successful differentiation among pristine planar crystals, rimed planar crystals, long columns, blocky columns, and g raupel. When a reasonable assumption about particle bulk density is made, q uantitative estimates of particle aspect ratios from radar depolarization d ata are in good agreement with in situ observations. Uncertainties of parti cle aspect ratios estimated from depolarization measurements due to 0.1 g c m(-3) variations in the assumed bulk density are about 0.1.