The performance of modified CCCma RTM in representing the Global Clear-sky Downwelling Shortwave Flux

The clear-sky total shortwave (SW, 0.3-5 μm), visible (VIS, 0.3-0.7 μm), and near-infrared (NIR, 0.7-5 μm) SW fluxes at the surface calculated by the low-spectral resolution version of the CCCma radiative transfer model (RTM) have been compared with the high-spectral resolution of MODTRAN6.0.2.5 (M6.0) calculations. The CCCma RTM was modified with four spectral bands: VIS (0.2- 0.69 μm), NIR1 (0.69-1.19 μm), NIR2 (1.19-2.38 μm), and NIR3 (2.38 - 5 μm), and used the same inputs of atmospheric profiles, AOD, surface albedo as M6.0. The computed total SW fluxes at the surface (SWDNsfc) from these two RTMs are then compared with the NASA CERES SYN1deg product, computed by the NASA Langley modified broadband Fu-Liou RTM. The global mean SWDNsfc are 246.5 W m-2 for M6.0, 246.4 W m-2 for CCCma, and 242.3 W m-2 for CERES SYN1deg product. The differences in SWDNsfc between three RTMs are remarkably low for global average, but with relatively large differences over the heavy dust and polluted regions, presumably due to different aerosol optical properties used in these RTMs. The assumption of lower SSA values used in CCCma is valid, which are responsible for higher VIS and lower NIR1 fluxes reaching the surface. The modified CCCma shows an excellent performance compared to M6.0, with very small differences in SWDNsfc, as well as across all four spectral bands. The different signs in "VIS and "NIR1 bands in comparison between CCCma and M6.0 result in the small differences in global total SW flux due to the cancelation. In addition to its accuracy, the modified CCCma RTM is also significantly faster than M6.0. This makes it an ideal choice for large-scale simulations where computationally efficiency is crucial.