The SPARC water vapour assessment II: profile-to-profile comparisons of stratospheric and lower mesospheric water vapour data sets obtained from satellites

Within the framework of the second SPARC (Stratosphere-troposphere Processes And their Role in Climate) water vapour assessment (WAVAS-II), profile-to-profile comparisons of stratospheric and lower mesospheric water vapour were performed by considering 33 data sets derived from satellite observations of 15 different instruments. These comparisons aimed to provide a picture of the typical biases and drifts in the observational database and to identify data-set-specific problems. The observational database typically exhibits the largest biases below 70 inline-formula>>hPa, both in absolute and relative terms. The smallest biases are often found between 50 and 5 inline-formula>>hPa. Typically, they range from 0.25 to 0.5 inline-formula>>ppmv (5 % to 10 %) in this altitude region, based on the 50 % percentile over the different comparison results. Higher up, the biases increase with altitude overall but this general behaviour is accompanied by considerable variations. Characteristic values vary between 0.3 and 1 inline-formula>>ppmv (4 % to 20 %). Obvious data-set-specific bias issues are found for a number of data sets. In our work we performed a drift analysis for data sets overlapping for a period of at least 36 months. This assessment shows a wide range of drifts among the different data sets that are statistically significant at the 2inline-formula>>σ uncertainty level. In general, the smallest drifts are found in the altitude range between about 30 and 10 inline-formula>>hPa. Histograms considering results from all altitudes indicate the largest occurrence for drifts between 0.05 and 0.3 inline-formula>>ppmv decade-1. Comparisons of our drift estimates to those derived from comparisons of zonal mean time series only exhibit statistically significant differences in slightly more than 3 % of the comparisons. Hence, drift estimates from profile-to-profile and zonal mean time series comparisons are largely interchangeable. As for the biases, a number of data sets exhibit prominent drift issues. In our analyses we found that the large number of MIPAS data sets included in the assessment affects our general results as well as the bias summaries we provide for the individual data sets. This is because these data sets exhibit a relative similarity with respect to the remaining data sets, despite the fact that they are based on different measurement modes and different processors implementing different retrieval choices. Because of that, we have by default considered an aggregation of the comparison results obtained from MIPAS data sets. Results without this aggregation are provided on multiple occasions to characterise the effects due to the numerous MIPAS data sets. Among other effects, they cause a reduction of the typical biases in the observational database.© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License.
The Atmospheric Chemistry Experiment (ACE), also known as SCISAT, is a Canadian-led mission mainly supported by the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada. We appreciate the HALOE Science Team and the many members of the HALOE project for producing and characterising the high-quality HALOE data set. We would like to thank the European Space Agency for making the MIPAS level-1b data set available. MLS data were obtained from the NASA Goddard Earth Sciences and Information Center. Work at the Jet Propulsion Laboratory, California Institute of Technology was done under contract with the National Aeronautics and Space Administration. SCIAMACHY spectral data have been provided by ESA. Maya Garcia-Comas was financially supported by the MINECO under its >Ramon y Cajal> subprogramme, project ESP2014-54362-P and EC FEDER funds. Stefan Lossow was funded by the >Stratospheric Change and its Role for Climate Prediction> (SHARP) under contract STI 210/9-2.