Comparison of TLS Temperatures

Results of the comparison of synthetic CHAMP TLS temperatures to MSU/AMSU records from UAH and RSS as well as to synthetic ECMWF and HadAT2 TLS temperatures are presented in Fig. 4 for the tropical region. SAC-C, GRACE, and COSMIC "anchor point" months are also shown, their TLS temperatures closely match CHAMP temperatures within the sampling error range of < 0.3 K (Fig. 4a). Regarding absolute TLS temperatures (Fig. 4a), ECMWF agrees best with RSS until January 2006, then it closely follows CHAMP due to improvement in ECMWF resolution. Global offsets relative to CHAMP are 0.11 K (±0.31 K) for UAH and -0.69 K (±0.16 K) for RSS. These different offsets can be explained by the different reference climatologies provided by UAH and RSS (for the calculation of absolute temperatures) due to different processing, calibration, and merging procedures. A detailed discussion is given in Steiner et al. (2007).

TLS temperature anomalies exhibit overall a very good agreement between UAH, RSS, ECMWF, and CHAMP in all analyzed regions for intra-annual variability (RMS of de-trended anomaly differences of the other data sets to CHAMP < 0.1 K globally, 0.1 K in the tropics, and < 0.25 K in the extratropics). Highest variability of all data sets occurs in the SH extratropics, where especially HadAT2 anomalies exhibit larger RMS differences (about a factor of two; factor of four in the SH extratropics due to sparse data).

Fig. 4 Synthetic TLS temperatures from CHAMP compared to real MSU/AMSU records from UAH (blue) and RSS (red), as well as HadAT radiosonde (green) records and ECMWF (yellow) shown for the tropical region. "Anchor point" months from SAC-C, GRACE, and COSMIC are marked with black dots. (a) Monthly-mean absolute TLS temperatures. (b) TLS temperature anomalies with respect to 2002-2005 average seasonal cycle. (c) Monthly mean TLS anomaly differences relative to CHAMP

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The anomaly differences of the UAH, RSS, HadAT2, and ECMWF to CHAMP RO are illustrated in Fig. 4c with the multi-year trends of UAH and RSS indicated as trend lines. Regarding multi-year trend differences, HadAT2 shows no appreciable difference from CHAMP trends. UAH and RSS exhibit a statistically significant (at >95% confidence level) cooling trend difference relative to CHAMP globally (-0.30 K/5 years and -0.36 K/5 years), with main contributions from the tropics (-0.40 K/5 years and -0.42 K/5 years), while in the extratropics the trend differences are not significant. The results in the tropics are basically consistent with the findings by Ho et al. (2007) showing values of -0.1 K (-0.2 K) between UAH

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