A new approach for simultaneously retrieving cloud albedo and cloud fraction from surface-based shortwave radiation measurements
Surface-based measurements of shortwave (SW) radiative fluxes contain valuable information on cloud properties, but have not been fully used to infer those properties. Here a new analytical approach is presented that simultaneously infers cloud albedo and cloud fraction from surface-based measurements of total and direct radiative fluxes. An inspection of the analytical formulation reveals that cloud fraction is primarily determined by the relative cloud radiative forcing for the direct radiation, defined as the difference between the clear-sky and all-sky direct downwelling radiative fluxes normalized by the clear-sky direct downwelling radiative fluxes, while cloud albedo is primarily determined by the ratio of the relative cloud radiative forcing for the total downwelling radiation to the relative cloud radiative forcing for the direct radiation. The new analytical approach is validated using synthetic measurements generated by the rapid radiative transfer model (RRTM) algorithm with known cloud inputs and some surface- and satellite-based measurements. The effect of cloud absorption is further corrected based on a suite of numerical experiments. The new approach demonstrates the utility of partitioning total radiation into direct and diffuse radiation, and eliminates the potential contamination of errors in existing approaches that retrieve cloud fraction and cloud albedo separately.