On the Interpretation of Shortwave Albedo-Transmittance Plots

The coefficients of linear regression lines fit to 1D model values of broadband surface absorptance T and top-ofatmosphere albedo " indicate the impact of clouds on atmospheric absorptance a. It is argued here that these coefficients cannot be compared directly to corresponding values for hourly observed data. This is because 1 " T = a for 1D models, but not for hourly observations. A 3D Monte Carlo radiative transfer algorithm and a realistic atmosphere are used to demonstrate that hourly data create an illusion of anomalous absorption by clouds on " vs. T plots. This is because of horizontal transport of radiation and inherently poor sampling of T. It would, however, be safer to compare 1D model regression coefficients to those for at least diurnal mean observations averaged over large grids containing several surface pyranometers.