Dual-path method and revised radiative model for retrieving cloud parameters from pairs of AVHRR images

An examination of AVHRR images obtained from the current triplet of nearly simultaneous NOAA polar orbiting satellites (namely, NOAA-9, -10, -12) revealed significant variances in the infrared radiation of opaque clouds observed from a satellite at different angles. To account for the effect of cloud temperature changes with depth, causing the essential part of these angular differences, the current cloud radiative model was revised following the principles of radiative transfer. Based on this model a dual-path method exploiting these differences to derive the cloud-top temperature and either the optical depth (of semitransparent clouds) or the ratio of effective lapse rate to volume absorption coefficient (of opaque clouds) was proposed. The derived cloud-top temperature and optical depth of semitransparent cirrus clouds were verified by the analysis of cloud shadows and albedos using visible technique. Comparison of the results obtained to the coincident retrieval by the split-window method demonstrates the advantages of the dual-path method. Also discussed are promising combinations of the proposed method with multichannel infrared and microwave methods.