Geometrically effective cloud fraction for solar radiation

It has been suggested in the past that an “effective” cloud fraction can be used to take into account cloud geometry effects in solar radiation parameterizations. All such models have been based on the reflected flux from non-plane-parallel cloud fields. In this study, it is shown that the “effective” cloud fraction based on absorption (or transmission) could differ considerably from that based on reflection and, moreover, is not constant throughout the solar spectrum even for fixed cloud geometry. Furthermore, the representation of cloud radiative forcing at the surface and at the top of the atmosphere in terms of cloud fractions are based on different “effective” cloud fractions. Therefore, a simple interpretation of measured values of the forcing ratio is not possible when geometric effects are present.