Ice Particle Orientation: Implication on Ice Cloud Remote Sensing with Sub-Millimeter Polarimetric Measurements

Ice clouds profoundly impact the Earth’s energy budget. However, the variable structure and high spatiotemporal coverage impose difficulties in characterizing their radiative characteristics and thus limit our understanding of the current climate and potential future changes. It continues to be a challenge for space-borne remote sensing techniques to retrieve ice cloud microphysical properties. Retrieving microphysical properties depends on macrophysical variables such as integrated ice mass (ice water path; IWP) and particle size distribution, but also particle shape and orientation. Previous studies have shown high-frequency sub-millimeter (sub-mm) channels to be valuable for inferring cloud IWP because of the significant scattering effect of ice clouds. It has also been demonstrated that incorporating polarimetric measurements may improve retrievals of ice cloud microphysics such as particle effective diameter (Deff). However, ice crystal orientations can significantly influence polarimetric measurements and subsequently can lead to errors in retrievals where ice particles are assumed to be randomly oriented.

Previously, we rigorously quantified the information content in the retrieval of ice cloud properties when ice particles are assumed to be randomly oriented. In this work, we continue to explore the feasibility of off-nadir polarized sub-mm channels to infer IWP and Deff simultaneously when some fraction of the cloud contains horizontally oriented particles. We investigate the sensitivity of polarization difference (difference in vertically and horizontally polarized brightness temperatures) to the varying fraction of horizontally oriented particles present at the top of the cloud. To get a robust characterization of the particle orientation effects on retrievals, we present an information content analysis in conjunction with a conventional retrieval error analysis.