PNNL

Abstract

In December 1999, NASA launched the Terra satellite. This platform carries five instruments that measure important properties of the Earth climate system. One of these instruments is the Multiangle Imaging Spectroradiometer, or MISR. This instrument measures light reflected from the Earth at a spatial resolution of 275-1100 m, at four wavelengths (446, 558, 672, and 866 nm), and at nine different viewing angles that vary from +70 to -70 degrees along the direction of flight [Diner et al., 2002]. These multiangle data have the potential to provide information on aerosols, surface, and cloud characteristics that compliments traditional single-view-direction satellite measurements. Before this potential can be realized, the accuracy of the satellite radiance measurements must be carefully assessed, and the implications of the radiometric accuracy on remote-sensing algorithms must be evaluated. In this article, we compare MISR multiangle measurements against two-dimensional (2-D) and 3-D radiative transfer calculations from an inhomogeneous cloud scene. Inputs to the radiative transfer code are based entirely on independently gathered data (ground-based radar, lidar, microwave radiometer, in situ aircraft data, etc.). The 2-D radiative transfer calculations compare favorably near nadir and in most of the forward scattering directions, but differ by as much as 10% in the backscattering directions. Using 3-D radiative transfer modeling, we show that this difference is due to the 3-D structure of the cloud deck, including variations in the cloud top height on scales less than 275 m, which are not resolved in the 2-D simulations. Comparison of the 2-D calculations to the MISR measurements, after accounting for the 3-D structure, show residual differences that are less than 4% at all angles at the MISR blue and green wavelengths. The comparison also reveals that the MISR measurements at the red and near-infrared wavelengths are too bright relative to measurements in the blue and green bands. On the basis of the results of this study, along with results from five other comparisons, the MISR calibration is being adjusted to reduce the red and nearinfrared Radiances.