Note on the definition of clear sky in calculations of shortwave cloud forcing

An important item to distinguish in estimations of cloud forcing is the characteristics of the "clear sky." In this study we investigate the influence of the composition of the clear sky in calculations of shortwave cloud forcing based on two case studies from the Monterey Area Ship Track Experiment (MAST). The forcing is calculated with respect to a clear sky devoid of aerosol particles and with respect to a clear sky containing the aerosol particles present in and below the cloud layer at below-cloud ambient humidity. It is found that in the case of a continentally influenced stratocumulus cloud containing a large concentration of dust and/or soot aerosols, the definition of clear sky makes an 8-10% difference in the upwelling solar irradiance and cloud forcing ratio.