A Cloud Fraction versus View Angle Technique for Automatic In-Scene Evaluation of the MISR Cloud Mask

The Multiangle Imaging Spectroradiometer (MISR), on board the Earth Observing System (EOS) satellite Terra, is the first high-resolution imager to make global, near-simultaneous multispectral and multiangle radiometric measurements of the earth. A standard product of MISR is the radiometric camera-by-camera cloud mask (RCCM), which provides a cloud mask for each of the nine MISR cameras. Validation of the RCCM is ongoing, and in this paper an automatic and efficient technique is described that is being used to flag scenes for which the quality of the RCCM may be suspect, thus allowing rapid convergence toward validation. The technique, herein called the F ␪ technique, makes use of the physical relationship that cloudiness increases with viewing obliquity. Where this behavior is not met for a given scene, the F ␪ technique flags the scene as potentially problematic. The technique is applied to ϳ4 months of MISR data to demonstrate its utility and to identify common problems that exist in version F01࿞0010 of the RCCM. In the course of research into the F ␪ technique, the existence of greater radiative and spatial contrast between clear and cloudy pixels in oblique views that measure radiation in the forward-scatter direction, as compared with oblique views that measure radiation in the backscatter direction, have been observed. As a result, thinner clouds can be detected in views that measure radiation in the forward-scatter direction as compared with oblique views that measure radiation in the backscatter direction for a given air mass. It is hypothesized that a similar effect must exist with other cloud detection techniques using radiative and spatial measures constructed from solar channels. It is shown that this effect manifests itself as a unique angular signature in the MISR RCCM that may be exploited to flag scenes as potentially being dominated by thin cirrus or a thick haze.