IMPROVED CLOUD DETECTION IN ALONG-TRACK SCANNING RADIOMETER (ATSR) DATA OVER THE OCEAN

Valid estimates of sea surface temperature (SST) from satellite data [ e.g., the Along Track Scanning Radiometer (ATSR)] are critically depen dent upon the identification and removal of cloud from the data, but f ew cloud-screening algorithms for ATSR data have appeared in the liter ature. A new algorithm, the ATSR Split-and-Merge Clustering (ATSR/SMC) algorithm, for cloud masking ATSR data Is presented which evaluates e very pixol in the image, is statistically reproducible, computationall y efficient, and requires no knowledge of cloud type. Moreover it is e ffective in detecting multilayer cloud structures in a scene, which is a difficult task because such systems generally have bimodal statisti cal distributions, It also accurately detects glint radiance, which is quite common in at least one of the 1.6 mu m views, subpixel cloud co ntamination near-cloud boundaries and low-lying marine stratiform clou d. Historically, these issues have interfered with ATSR-based SST retr ieval [see the work of Jones et al., (1996a,b) and the references cite d therin]. The SSTs derived from. the cloud-free ocean pixels were val idated with 96 buoy observations and the mean difference (buoy-SST) wa s +0.24 degrees C+/-0.51 degrees C. For the 103 pails of images (forwa rd/nadir views) tested, the mean 11 mu m BTs that result from SADIST ( standard ATSR processing) cs. ATSR/SMC cloud detection are 0.4 degrees C (daytime) and 0.6 degrees C (nighttime) colder for SADIST than for ATSR/SMC, even though the SADIST cloud masks generally overdetect clou ds relative to ATSR/SMC cloud masks. These results, plus others discus sed in the test, support the conclusion that the new procedure produce s cloud masks which are superior to the standard ATSR operational, clo ud mask product and it retains substantially more valid pixels. The al gorithm can be used in tropical and midlatitude regions. It is not des igned to detect sea ice, and consequently should not be used in polar regions. Finally, the approach call easily be adapted to ATSR-2 data a nd to other darn to be taken from soon to be launched sensors. (C) Els evier Science Inc., 1998.