Introduction

Long term upper air temperature records have been established from radiosonde observations (Lanzante et al. 2003; Sherwood et al. 2005; Thorne et al. 2005b) and from satellite observations with the Microwave Sounding Unit (MSU) and the Advanced MSU (AMSU) (Christy et al. 2003; Grody et al. 2004; Christy and Spencer 2005; Mears and Wentz 2005; Vinnikov et al. 2006). These temperature trend series now show convergence toward surface warming estimates and provide a better agreement with climate models (Santer et al. 2005; Karl et al. 2006). Still significant divergence remains in their long-term large-scale mean trends (Seidel et al. 2004; Thorne et al. 2005a). Most likely radiosonde data underestimated tropical mean temperature trends (cooling trend bias) (McCarthy et al. 2008).

In this context the radio occultation (RO) technique offers new possibilities by providing an independent type of high quality observations of the atmosphere. The validation and climatological exploitation of RO data from GPS/MET (Schr0der et al. 2003) and CHAMP (e.g., Schmidt et al. 2004; Gobiet et al. 2005; Borsche et al. 2007) indicated the suitability of RO data for global climate monitoring and modeling (Leroy et al. 2006; Foelsche et al. 2008a, 2009a,b). Steiner et al. (2007) carried out a comparison of CHAMP RO data with MSU/AMSU and equivalent radiosonde data sets and found significant trend differences to MSU/AMSU TLS data in the tropical region which has also been confirmed by Ho et al. (2007). In the current study we analyze in detail respective error sources regarding the retrieval of RO data (potential initialization biases, dry temperature assumption), the building of RO climatologies (sampling error) and the computation procedure of MSU equivalents (static weighting function versus radiative transfer model).

A short description of the respective data sets is given in Sect. 2 and the comparison setup is described in Sect. 3. The results of the study are presented and discussed in Sect. 4. Section 5 summarizes our findings and conclusions and gives a brief outlook to further investigations.

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