On the Earth's surface energy exchange determination from ERS satellite ATSR data. Part I: Long-wave radiation

This is the first in a series of papers which addresses the determination of the Earth's surface energy exchange using data from the Along-Track Scanning Radiometer (ATSR). This paper focuses on long-wave radiation from sea and land surfaces and a technique is proposed for the derivation of land surface temperature (LST) and land surface emissivity retrieval using ATSR data in a new simultaneous split-window method. Two points regarding net long-wave radiation are also considered. Firstly, over land and sea, differences in several previously published are discussed. Secondly, over sea, the effect on the net longwave radiation of using sea surface skin temperatures, which can be derived accurately from satellite thermal band data, as input to the empirical formulae is compared to the use of bulk water temperature taken from in situ measurements. Finally, a new formula is developed for the calculation of net long-wave radiation at the surface. The equivalent sky temperature, T-sky, is used and the results agree with those obtained using the Oberhuber formula. Both of these formulae are useful for the calculation of net long-wave radiation over wet areas such as sea, with a high relative humidity. Initial tests of the formulae were carried out using ATSR sea surface temperatures (SSTs) and LSTs in the United Kingdom. The formulae were also tested using TOGA-TAO data and ATSR sea surface temperatures over the Pacific Ocean. From our results, the net long-wave radiation showed the magnitude and spatial variability to be (15-35 W m(-2)). The absolute difference of net long-wave radiation by using ATSR SST and TOGA SST is around 3 W m(-2) for most areas, but the maximum difference is up to 7 W m(-2). The relative difference is more than 10% and up to 30%.