GLOBAL REMNANT CLOUD CONTAMINATION IN THE ALONG-TRACK SCANNING RADIOMETER DATA - SOURCE AND REMOVAL

Validation studies indicate that in cloud free conditions the along-tr ack scanning radiometer (ATSR) on the ERS 1 satellite is measuring glo bal sea surface temperature (SST) to a point relative precision of bet ter than 0.3 degrees C. However, recent reports show that the presence of nighttime remnant cloud contamination is affecting ATSR SST precis ion in certain geographical areas. We modify the filtering scheme desc ribed by Jones ct al. [1996] for removing remnant cloudiness in ATSR 0 .5 degrees spatially averaged sea surface temperature (ASST) data from the South Atlantic and apply this scheme to global ASST data (version 500) for the whole of 1992, 1993, and 1994. We find that globally 4.6 5% of the ASST data is cloud contaminated and that remnant cloud remov al substantially reduces differences between day and night SST signals . The cloud contamination is seasonal and is linked to the occurrence of marine stratiform (fog, stratus, and stratocumulus) cloud types. Su ch cloud is low-lying and uniform and thus difficult to detect with co nventional cloud-clearing techniques. Our effective global removal of remnant cloudiness will improve the accuracy of the ASST data and thus benefit the use of the ATSR in the early detection of climate change.