OSIRIS Subvisual Cirrus Cloud Observations and Modeling

The OSIRIS instrument on the Odin satellite routinely observes subvisual cirrus clouds as enhancements to its limb-scattered sunlight profiles. Complementary views are provided by its subsystems: a vertically-mounted set of InfraRed Imaging pixels and a limb-scanning Optical Spectrograph that operates between 280 and 810 nm. By using the successive-orders radiative transfer model SASKTRAN, which operates in a fully spherical geometry, the optical properties of these clouds are estimated. To provide this modeling capability over a large range of particle sizes, SASKTRAN employs T-matrix and Discrete Dipole Approximation algorithms, together with an in-situ based set of cirrus scattering properties. Because this scattering involves very large size parameters with extremely asymmetric scattering, SASKTRAN is equipped with photon conservation and phase function truncation techniques. Since OSIRIS has been in continuous operation since 2001, these observations provide an excellent long-term record of cloud properties. In this poster, climatological behaviours of observed clouds and optical properties inferred from spectral modeling of these clouds are presented.