TOWARDS BETTER QUALITY OF AVHRR COMPOSITE IMAGES OVER LAND - REDUCTION OF CLOUD CONTAMINATION

A procedure for reducing cloud contamination in NOAA/AVHRR composite i magery is proposed and applied to the NOAA global vegetation index dat a set. The suggested approach is based on thresholding the AVHRR/Chann el 4 (11 mu m) brightness temperature. The global spacetime-angle depe ndent thresholds (monthly, 2 degrees x2 degrees, for 30 degrees-viewin g angle bins) are estimated in two steps. First, a clear-sky backgroun d for Channel 4, in terms of means and standard deviations, is develop ed from GVI weekly composite data using their ''greenest'' subsamples. Second, this background is used to estimate the space-time-angle depe ndent thresholds. Both the size of the ''greenest'' subsample for deri ving the climatology and its use for constructing the thresholds are d iscussed in detail using targets in central United States and Thailand . The approach is generalized further and applied globally. Special at tention is devoted to the development of quantitative criteria to esti mate the efficiency of both steps and to the improvement resulting fro m implementation of the proposed procedure of cloud screening.