Comparison of Line-by-Line Equivalent Radiative Transfer Model and Moderate-Resolution Transmission Model for accuracy assessment of algorithms

To support the verification and implementation of the National Polar-Orbiting Operational Environmental Satellite Systemʹs Visible-Infrared Imaging-Radiometric Suite (VIIRS) algorithms used for inferring cloud environmental data records, an intercomparison effort has been carried out to assess the consistency between the simulated cloudy radiances-reflectances from the University of California at Los Angeles Line-by-Line Equivalent Radiative Transfer Model 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 found discrepancies in the visible and near-infrared reflectances from the two models, which presumably are due to the difference in phase function (nonspherical versus Henyey-Greenstein),different numbers of phase function expansion terms (16 versus 200 terms), and different treatment of forward peak truncation in each model. Using the MODTRAN4, we also found 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 affects the prospective test plan for the VIIRS cloud algorithms but also should lead to improvements in future MODTRAN releases.