Back-Scatter by Oblate Ice Spheroids

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  • 1 Geophysics Research Directorate, Air Force Cambridge Research Laboratories
  • | 2 Allied Research Associates, Inc., Concord, Mass
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Abstract

Back-scatter measurements are presented for a set of oblate ice spheroids and a set of ice-simulating dielectric spheroids at both 3.2- and 9.7-cm wavelengths as a function of aspect and polarization. The data are highly complex and not subject to simple generalizations. The only regular behavior is shown by particles whose major axes are smaller than about λ/4. This is approximately the upper size limit to which the simple Gans theory is applicable. For these small particles the cross section is a function only of the maximum linear dimension in the direction of polarization; thus, for polarization parallel to the major axis, the back-scatter is independent of aspect. However, as the particle axis/diameter (b/a) ratio decreases, the cross section relative to an equivolume sphere increases. For polarization perpendicular to the major axis, the cross section decreases from a maximum on the large face to a minimum on the small face. AS b/a decreases, the small face cross section continually decreases below that of an equivolume sphere.

For D/λ≅0.5, large (circular) face cross sections are generally enhanced (with respect to equivolume spheres); small face cross sections are usually degraded, the more so the smaller b/a. The situation is reversed only for b/a>0.8. As with the smaller particles, polarization parallel (to the major axis) is superior to perpendicular polarization. When D/λ>0.5 the large face cross sections tend to follow that indicated by geometric optics, with enhancement usually occurring for b/a>0.7, but oscillating between enhancement and degradation for smaller b/a's. Small face cross sections are usually equal to or less than those of equivolume spheres in this size regime, and in contrast to the small sizes, perpendicular polarization is superior to parallel for b/a>0.5. In exceptional cases, the small face cross sections are enhanced, but only by a few decibels. Thus, unless wetting effects are important, it is not possible to attribute extraordinarily large reflectivities of hailstorms to shape and orientation phenomena.

The cancellation ratios to be expected upon switching from linear to circular polarization cover a broad range, overlapping that found for rainfall. Thus, polarization switching is not likely to provide a general means of distinguishing dry, oriented, oblate hail from rain. When the particles are water coated, polarization switching would be more useful. However, any tendency toward random orientation would diminish the polarization effects.

Abstract

Back-scatter measurements are presented for a set of oblate ice spheroids and a set of ice-simulating dielectric spheroids at both 3.2- and 9.7-cm wavelengths as a function of aspect and polarization. The data are highly complex and not subject to simple generalizations. The only regular behavior is shown by particles whose major axes are smaller than about λ/4. This is approximately the upper size limit to which the simple Gans theory is applicable. For these small particles the cross section is a function only of the maximum linear dimension in the direction of polarization; thus, for polarization parallel to the major axis, the back-scatter is independent of aspect. However, as the particle axis/diameter (b/a) ratio decreases, the cross section relative to an equivolume sphere increases. For polarization perpendicular to the major axis, the cross section decreases from a maximum on the large face to a minimum on the small face. AS b/a decreases, the small face cross section continually decreases below that of an equivolume sphere.

For D/λ≅0.5, large (circular) face cross sections are generally enhanced (with respect to equivolume spheres); small face cross sections are usually degraded, the more so the smaller b/a. The situation is reversed only for b/a>0.8. As with the smaller particles, polarization parallel (to the major axis) is superior to perpendicular polarization. When D/λ>0.5 the large face cross sections tend to follow that indicated by geometric optics, with enhancement usually occurring for b/a>0.7, but oscillating between enhancement and degradation for smaller b/a's. Small face cross sections are usually equal to or less than those of equivolume spheres in this size regime, and in contrast to the small sizes, perpendicular polarization is superior to parallel for b/a>0.5. In exceptional cases, the small face cross sections are enhanced, but only by a few decibels. Thus, unless wetting effects are important, it is not possible to attribute extraordinarily large reflectivities of hailstorms to shape and orientation phenomena.

The cancellation ratios to be expected upon switching from linear to circular polarization cover a broad range, overlapping that found for rainfall. Thus, polarization switching is not likely to provide a general means of distinguishing dry, oriented, oblate hail from rain. When the particles are water coated, polarization switching would be more useful. However, any tendency toward random orientation would diminish the polarization effects.

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