Optical backscattering from near-spherical water, ice, and mixed phase drops.

An experimental assessment of the scattering behavior of freely falling artificial raindrops and mechanically suspended drops in the ice and mixed phase has been undertaken with a device which simultaneously measures the parallel and cross polarized components of backscattered linearly polarized laser light (6328 A). Among the findings are that linear depolarization ratios (delta) are generally <0.01 for raindrops up to nearly 6-mm diam, near 0.5 for regularly shaped frozen drops, and between 0.35 and 1.0 for more irregular ice particles. Anomalous scattering behavior has been observed during the liquid to solid drop phase transition (delta > 1.0) and in the relatively great amounts of parallel polarized energy returned from raindrops >~4 mm. Backscattered signal variations produced during drop melting reveal that delta values tend to remain near the initial ice value until most of the ice has changed phase. The details of the variations aid in the determination of the dominant scattering mechanisms responsible for the b ckscatter from large, near-spherical particles. The results are shown to have some bearing on measurements of atmospheric hydrometeors obtained by lidar.