Quantifying Cloud-Induced Shortwave Absorption: An Examination of Uncertainties and of Recent Arguments for Large Excess Absorption

Abstract

The quantification of cloud-induced shortwave atmospheric absorption is a painstaking task and often the subject of contention. Several analytical methods previously used for this purpose are examined in detail applying each method to a set of collocated satellite and surface measurements of radiant fluxes taken in April of 1994 in Oklahoma. It is demonstrated that, if care is not taken, conclusions regarding cloud-induced absorption can be as much a function of the chosen analytical methods as they are of the data themselves. It is argued that the best method for determining the cloud radiative forcing ratio is from the slope of a plot of the cloud radiative forcing ratio at the surface versus the cloud radiative forcing at the top of the atmosphere and/or a normalized analog. Application of this method shows that clouds in Oklahoma, on average, induced an absorption of 4% more of the solar insulation than did clear sky. An examination is made of three recent papers that have reported cloud-induced atmospheric absorption in large excess over that which has been generally considered possible. It is shown that, once uncertainties and biases in the analytical methods are considered, the results of all three papers are consistent with conventional formulations of cloud-radiation interactions.