Evaluation of clear-sky surface downwelling shortwave fluxes computed by three atmospheric radiative transfer models

In this study the clear-sky total, direct, and diffuse shortwave (SW) fluxes at the surface, have been calculated by three radiation transfer models (RTMs) - MODTRAN6.0 (M6.0), Canadian Centre for Climate Modelling and Analysis (CCCma), and Langley-modified Fu-Liou (NASA CERES). These calculations have been evaluated by surface measurements collected from seven sites that represent different climatological regimes with various surface scene types including ocean, grassland/continental, desert, and snow/sea ice. For pristine atmospheric conditions, SW fluxes predicted by CCCma and M6.0 shows little variation, which lays a baseline for further analysis. Note that computing time required by CCCma is similar to 1000 times smaller than M6.0. Based on all samples collected from seven sites, mean differences of total, direct, and diffuse fluxes between surface measurements and CCCma / M6.0 / Fu-Liou are [5.3 / 2.4 / 0.9], [-2.2 / -5.1 / -13.7], and [7.5 / 7.5 / 14.6] W m(-2), respectively. Histograms of differences between the three RTM calculations and surface measurements show that CCCma computed direct and diffuse fluxes have the smallest biases with standard deviations similar to those for M6.0, while Fu-Liou values have the largest biases and standard deviations. While Fu-Liou outperforms for total flux, especially for desert conditions, it is hampered by large biases for direct and diffuse across all scene types. The three RTMs are consistent with showing the least error for total flux and the largest in diffuse based on bias, correlation, and root mean square error.