THE AATSR LAND SURFACE TEMPERATURE (LST) PRODUCT: COMPARISON WITH LSTS FROM SEVIRI

The Advanced Along-Track Scanning Radiometer (AATSR) onboard ESA’s Envisat Satellite, is the third in a series of precision radiometers designed to measure Sea Surface Temperature (SST). Since March 2004, an operational Land Surface Temperature (LST) product has also been available, which is the first of its kind since the (A)ATSR mission began in 1991. This product utilises a nadir-only split-window algorithm, with a target accuracy of 2.5 K during the day, and 1.0 K at night. Initial validation of the retrievals over a number of world-wide sites demonstrates that the product is achieving these accuracies, or better, under most cloud-free conditions. We present an overview of the operational AATSR LST product algorithm and the AATSR LST validation programme to date, and the results of some preliminary comparisons with equivalent data from the Spinning Enhanced Visible and Infra-red Imager (SEVIRI) instrument, over select sites in Europe. The results of the comparisons demonstrate that the SEVIRI LSTs are typically 1-2 K (standard deviation 1-3 K) colder than those from the AATSR. For most sites, the magnitude and standard deviation of these differences is lower at night. The results also indicate that the observed biases between the AATSR and SEVIRI LSTs appear to be seasonal, where the minima of the cycles (SEVIRI minus AATSR) occur during the summer months. A similar seasonal bias is observed in AATSR LST data in a two-year validation experiment over a site in Oklahoma, North America. In this study, we investigate the possibility that the AATSR LST retrievals are seasonally biased through radiative transfer simulations. The results of this theoretical study indicate that the AATSR retrievals, at least in part, are subject to such biases, where the principal source is the algorithms dependency on atmospheric conditions prevalent at the time of the satellite overpass. 1