Polarimetric signatures from Ice Crystals observed at 95 GHz in Winter Clouds. Part II: Frequencies of Occurrence

Abstract

Data are presented, from a large collection of observations in wintertime clouds in Wyoming, which show that the fraction of cloud volumes from which significant radar polarimetric information can be obtained is small. For example, when averaged over all available samples, signals exceeding the chosen limits of 3 dB for ZDR and -18 dB for linear depolarization ratio were found in just a few percent of the observations for radar beam incidence angles of less than 45°. In general, the polarimetric signatures are interpreted as indicators of the prevalence of pristine and lightly rimed crystals, as opposed to more densely rimed crystals, graupel, or aggregates. However, specific cases are presented to illustrate exceptions to this interpretation. The polarimetric signatures provide information regarding ice crystal types from larger cloud volumes than can be observed with in situ probes, and thus may aid in understanding the evolution and possible origin of hydrometeors in the clouds. They may also help to refine assumptions made in the modeling of radiative transfer through clouds.