Lower stratospheric temperatures from CHAMP RO compared to MSU/AMSU records: An analysis of error sources

Abstract

A comparison of lower stratospheric temperatures (TLS) from (Advanced) Microwave Sounding Unit (MSU/AMSU) records with CHAMP radio occultation (RO) data was performed for September 2001-December 2006. Synthetic TLS temperatures were calculated by applying global weighting functions to monthly RO temperature climatologies and alternatively by applying the RTTOV-8.5 radiative transfer model to the individual CHAMP RO profile data set. The results published by Steiner et al. (2007) showed very good agreement of CHAMP TLS anomalies with MSU records from the University of Alabama in Huntsville (UAH, USA) and from Remote Sensing Systems (RSS, USA) for intra-annual variability. Statistically significant trend differences of UAH and RSS with respect to CHAMP TLS anomalies were found in the tropics (-0.40 K/5 years to -0.42 K/5 years). In this context error sources regarding the retrieval of RO data, the building of climatologies, and the related synthetic MSU computation procedure were analyzed and found to be of minor importance. In the NH extratropics the TLS specific sampling error showed a significant negative trend which is projected globally but tropical trends are small. In total, the contribution of errors from RO was found to be about an order of magnitude smaller than the trend differences and thus insufficient to account for them, especially in the tropical region. Their resolution, currently pursued in a further study, requires either the presence of currently unresolved biases in the MSU records or additional, so far overlooked, sources of error in the RO TLS record. SAC-C, GRACE, and COSMIC TLS temperatures were found to closely match CHAMP temperatures. The results underpin that inter-comparison of independent estimates of the same variable from different instruments is beneficial for the detection of residual weaknesses in otherwise high quality climate records. © 2009 Springer-Verlag Berlin Heidelberg.