GLOBAL VALIDATION OF THE ALONG-TRACK SCANNING RADIOMETER AGAINST DRIFTING BUOYS

The along-track scanning radiometer (ATSR) was launched on the Europea n Space Agency's first remote sensing satellite, ERS 1, on July 17 199 1. ATSR is designed to retrieve sea surface temperature (SST) to an ac curacy of 0.25 K rms, which represents more than a factor of 2 improve ment over any previously flown satellite radiometer. Early validation studies from limited regions suggest that ATSR is capable of measuring SST to near this design accuracy. We report a global validation study against quality-controlled drifting buoys by examining 280 matchups w orldwide with ATSR measurements at their full (1 km) resolution. We in vestigate optimizing the precision of ATSR using four different SST al gorithms derived using a theoretical atmospheric transmission model, c ombined with various techniques to reduce remnant noise and other erro rs. We find that a ''low-noise'' retrieval algorithm incorporating onl y the 3.7 and 11 mu m nadir view channels gives the optimum precision, a global pixel precision of 0.26 K (or 0.25 K if 1/2 degrees spatial averages are used). A standard deviation of 0.25 K against global drif ting buoy data approaches the geophysical limit set by the inherent va riability of the skin effect and by the buoy bulk temperature accuracy . Further progress will require comparison against quality in situ rad iometer-derived skin temperatures, although the problem of obtaining s ufficiently large and diverse data sets will need to be addressed.