Modeling the Angular Anisotropy in Radiance Observed by MISR over Polar Clouds and Surfaces

Observations from the Multi-angle Imaging SpectroRadiometer (MISR), on-board EOS-Terra, show that clouds and snow/ice surfaces have very different angular signatures in the visible and near-IR part of the electromagnetic spectrum. In particular, clouds appear much brighter than the snow/ice surface for oblique MISR cameras viewing forward-scattered radiation. These oblique measurements suggest that clouds over snow/ice have a large impact on upward scattered radiation, in spite of appearing similar to snow/ice in nadir satellite observations.

MISR provides a unique opportunity to test radiative transfer models applied to polar clouds and surfaces. Past studies have relied only on nadir satellite measurements, in which clouds and the surface appear very similar. In fact, our results show that clouds can appear either brighter or darker than the surface when viewed at nadir, depending on the cloud optical depth, surface albedo, and solar zenith angle. However, the scattering properties of clouds are far more discernible from clear sky at more oblique measuring angles. The nine camera angles of MISR provide an angular constraint to verify the behavior of radiative transfer models applied to polar conditions.

This presentation will focus on comparisons between MODTRAN and Streamer simulations and MISR observations. We will demonstrate that even simple liquid clouds and Lambertian surfaces in these models mirror the angular trends in MISR radiances for many situations, although the differences between model and MISR are a strong function of view angle. We will then show more realistic results by using popular snow-surface models to more accurately portray the surface. We will also introduce the transition albedo for nadir and oblique angles, defined as the point where clouds and the surface appear equally bright, and demonstrate how it varies with such factors as solar zenith angle and cloud optical depth.