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 ne arly simultaneous NOAA polar orbiting satellites (namely, NOAA-9, -10, -12) revealed significant variances in the infrared radiation of opaq ue clouds observed from a satellite at different angles. To account fo r the effect of cloud temperature changes with depth, causing the esse ntial part of these angular differences, the current cloud radiative m odel was revised following the principles of radiative transfer. Based on this model a dual-path method exploiting these differences to deri ve the cloud-top temperature and either the optical depth (of semitran sparent clouds) or the ratio of opaque lapse rate to volume absorption coefficient (of opaque clouds) was proposed The derived cloud-top tem perature and optical depth of semitransparent cirrus clouds were verif ied by the analysis of cloud shadows and albedos using visible techniq ue. Comparison of the results obtained to the coincident retrieval by the split-window method demonstrates the advantages of the dual-path m ethod. Also discussed are promising combinations of the proposed metho d with multichannel infrared and microwave methods. (C)Elsevier Scienc e Inc., 1997.