The effect of clouds on atmospheric absorption of solar radiation

It has long been an issue of whether clouds enhance or reduce atmospheric absorption of solar radiation. Radiative transfer calculations show that the effect of clouds on atmospheric absorption of solar radiation depends strongly on the solar zenith angle, cloud height and surface albedo. Over wide ranges of these parameters, clouds have the effect of reducing atmospheric solar heating. This result can be altered if the cloud specific absorption is amplified by a factor of 5. For the ratio of cloud forcing at the surface to that at the top of the atmosphere to reach 1.5, as suggested by recent observational analyses, cloud specific absorption would be required to increase by a factor of ≈40 beyond what has been determined by in situ aircraft measurements. Since there is no observational evidence to support such strong absorption, the excess cloud absorption, if it occurs, can only be explained by some unknown factors other than the cloud specific absorption.