Sensitivity of Radiative Forcing to Variable Cloud and Moisture

Abstract

The influence of cloud and moisture distribution on the radiative forcing of the atmosphere is investigated. A simple radiative transfer model is developed to calculate the radiative fluxes at the boundaries of the atmosphere and the net radiative absorption of the atmospheric column. It is of a type and simplicity which is compatible with the present climate models and is shown to compare well with more complicated multiple-scattering calculations. Details of the atmospheric temperature and moisture structure and its radiation properties are systematically relaxed to test the assumption associated with even simpler models. The radiative properties of the atmosphere are shown to be strong functions of cloud amount, cloud type, latitude and atmospheric state. Results are presented which have several implications regarding climate modeling. It appears that radiative schemes in large-scale models should take into account full vertical and horizontal variability of moisture and cloud as minimum requirements for an adequate determination of the radiative forcing. Such a requirement emerges from calculations which show that the longitudinal gradient of radiative heating is of the same magnitude as the latitudinal gradient. It is shown that much of the longitudinal variation is due to the cloud distribution. The application of schemes using only zonally averaged cloud distributions is questioned. The applicability of various radiation schemes used in simple energy balance models is discussed, particularly with regard to their use in investigating the impact of changes in CO² and insolation on climate. It is shown that the radiative forcing of the atmosphere is extremely sensitive to variations in the state of the atmosphere and radiative parameters. It is suggested that small errors in the estimate of such quantities as emissivity may render models incapable of inferring changes which may occur in the real atmosphere.