Improvements in the Derivation of Bulk Scattering Properties for Ice Clouds at Visible Through Far-Infrared Wavelengths

This work focuses on improvements to our methodology for building both spectral and narrowband bulk scattering optical models appropriate for satellite imagers and hyperspectral infrared (IR) sensors. These ice cloud bulk scattering models are based on (a) a comprehensive set of microphysical models developed from in situ measurements of ice clouds, and (b) a set of ice particles used for light scattering calculations that include droxtals, columns, plates, bullet rosettes, and aggregates of columns or plates. For this work, in situ measurements are now available from a variety of field campaigns, including ARM-IOP, CRYSTAL-FACE, ACTIVE, SCOUT, MidCiX, and pre-AVE. Additionally, the light scattering calculations have been improved by including the full phase matrix as well as incorporating a new treatment of forward scattering that now obviates the use of a delta-transmission term. Furthermore, the single-scattering properties now include surface roughness. A library of single-scattering properties now encompasses a wide range of particle sizes, where the maximum dimension ranges from 2 to 10000 microns, and wavelengths, from 0.3 to 100 microns. Calculations are now available for hollow bullet rosettes as well as aggregates of plates.

The bulk scattering models discussed include applications to imagers such as MODIS and IR interferometers such as the Atmospheric Infrared Sounder (AIRS). Since those models have become available, various efforts have been made to intercompare inferred ice cloud properties from various sensors that are part of the Earth Observing System (EOS) A-Train constellation. For example, ice cloud extinction (i.e., optical thickness) is being compared between MODIS and CALIOP, the lidar on the CALIPSO platform. There are differences that need further explanation; one line of thought is that inclusion of surface roughness would move the MODIS optical thickness closer to that obtained by CALIOP. In a different comparison of ice cloud products between POLDER (POLarization and Directionality of the Earths Reflectances) and MODIS, we find that differences in the single-scattering properties for the assumed ice particles can lead to quite different values of ice cloud optical thickness and effective particle size. The results from these and other comparisons have led us to re-evaluate and improve certain aspects of the current ice cloud bulk scattering models.