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Comparison of the UCLA-LBLE radiative transfer model and MODTRAN for accuracy assessment of the NPOESS-VIIRS cloud optical property algorithms

To support the verification and implementation of the Visible/Infrared Imaging/Radiometric Suites algorithms used for inferring cloud environmental data records, an inter-comparison effor has been carried out to assess the consistency between the simulated cloudy radiances/relectances from the University of California at Los Angeles line-by-line equivalent radiative transfer model (UCLA-LBLE RTM) and those from the Moderate-Resolution Transmission Model (MODTRAN) with the 16-stream Discrete Ordinate Radiative Transfer Model (DISORT) incorporated. For typical ice and water cloud optical depths and particle sizes, we find discrepancies in the visible and near-infrared reflectances from the two models, presumably due to the difference in phase function (non-spherical vs. Henyey-Greenstein), different numbers of phase function expansion terms (16-term vs. 200-term), and different treatment of forward peak truncation in each model. Using MODTRAN4, we also find substantial differences in the infrared radiances for optically thick clouds. These differences led to the discovery by MODTRAN4 developers of an inconsistency in the MODTRAN4/DISORT interface. MODTRAN4 developers corrected the inconsistency, which provided dramatic reductions in the differences between the two radiative transfer models. The comparison not only impacts the prospective test plan for the VIIRS cloud algorithms, but also leads to improvements in future MODTRAN releases.