ABSORPTION OF SOLAR-RADIATION BY STRATOCUMULUS CLOUDS - AIRCRAFT MEASUREMENTS AND THEORETICAL CALCULATIONS

Aircraft observations of shortwave radiative properties of stratocumul us clouds were carried out over the western North Pacific Ocean during January 1991. Two aircraft were equipped with a pair of pyranometers and near-infrared pyranometers. Downward and upward shortwave fluxes a bove and below the cloud were synchronously measured by two aircraft. The cloud radiative properties, especially the absorptance obtained fr om measurements, were compared with those calculated. Aircraft measure ments and Monte Carlo calculations showed that spatial inhomogeneities of clouds cause horizontal radiative convergence and divergence, and that vertical radiative convergence-that is, absorptance with a usual definition-apparently becomes extremely large or negative. The apparen t absorptance could be corrected by a method that evaluates the true a bsorption from the difference between the apparent visible and near-in frared absorptions. The corrected absorptance agreed well with the the oretical absorptance calculated with plane-parallel cloud models. It i s also inferred that the anomalous absorption pointed out by aircraft observations in previous studies does not exist.