As a validity test we alternatively applied the state-of-the-art RTTOV8.5 radiative transfer model (Saunders and Brunel 2004; Saunders et al. 2006) to the individual CHAMP RO profile data set for the whole time period under study. The RTTOV_8.5 model was available from the MetOffice (Exeter, UK) at http://www.metoffice.gov.uk/research/interproj/nwpsaf/rtm/. Radiative transfer models account for land/sea differences in surface emissivity and for variations in atmospheric temperature and moisture as a function of space and time. According to Santer et al. (2000) the two methods agree best in the lower stratosphere, since the TLS temperatures are largely insensitive to tropospheric and surface variability. They found trend differences of < 0.01 K/decade globally and < 0.02 K/decade on hemispheric scales arising from the choice of method for computing equivalent MSU temperatures (Santer et al. 2000) which has been confirmed by the results of the current study (see Sect. 4.2).
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