What can we learn about ice microphysics from sub-mm polarimetry?

Abstract:

The increased availability of microwave scattering databases for realistic ice particles in the past decade has led to improved retrievals of ice-phase cloud and precipitation, routine all-sky data assimilation, and spurred the next generation of active and passive remote sensing platforms for ice cloud and precipitation observation. In particular, the extension of these databases to the mm/sub-mm frequencies (defined here as 200-1000 GHz) has enabled simulation/retrieval from airborne sub-mm instruments such as International Submillimetre Airborne Radiometer (ISMAR) and Configurable Scanning Sub-millimeter Instrument/Radiometer (CoSSIR), and simulation of observations from future sensors such as the Ice Cloud Imager (ICI). Furthermore, the availability of scattering properties for some particles at preferred alignment ("azimuthally-random" orientations) has provided the capability to examine the sensitivity of polarized sub-mm measurements to ice habit and orientation distribution.

Using high-resolution, multi-species, multi-moment cloud-resolving model output, we performed a series of sensitivity tests to the orientation distribution of cloud ice (represented by hexagonal plates) and aggregate snow for the full range of CoSSIR frequencies (4 channels near 183 GHz, 3 channels near 325 GHz, and 684 GHz, all with dual polarization) and incidence angles (0-60 degrees). These tests reveal that the lower-frequency channels, and channels near the wings of absorption lines, are mostly sensitive to the orientation distribution of aggregates, whereas the channels near the absorption line center frequencies and at higher frequencies are sensitive to the cloud ice orientation distribution as well. Orientation distributions have a significant impact on near-nadir brightness temperatures (>10 K) and radar reflectivity (> 3 dBZ) at Ku-, Ka-, and W-band as well. These results demonstrate the benefit of dual-polarization measurements and need for constraints on the orientation distribution to understand current and future radar and sub-mm measurements of ice clouds and precipitation.