VALIDATION OF A SATELLITE CLOUD RETRIEVAL DURING ASTEX

An algorithm using NOAA-12 AVHRR (Advanced Very High Resolution Radiom eter) solar reflectance measurements for retrieving cloud droplet size and optical thickness has been applied to a boundary layer stratocumu lus cloud in the vicinity of the Azores on 12 June 1992 during the Atl antic Stratocumulus Transition Experiment (ASTEX). This day was partic ularly advantageous for validations because of the absence of cirrus o r other higher-level clouds during the satellite overpass and the exis tence of a large relatively uniform stratus cloud deck. Uncertainty es timates for the retrievals are presented dong with a discussion of the algorithm. An in-flight absolute calibration of AVHRR channel 1, nece ssary for accurate optical thickness retrievals, was done concurrently with the retrievals through comparison with a visible radiometer flow n on the National Aeronautics and Space Administration's ER-2 and usin g the stratus cloud as the common reflectance target. Results are comp ared with in situ microphysical measurements taken with the Particulat e Volume Monitor (PVM-100) and Forward Scattering Spectrometer Probe ( FSSP-100) instruments on the University of Washington C-131A aircraft. Satellite retrievals of both optical thickness and droplet size lie w ithin the values measured by the two in situ instruments.