The SPARC water vapour assessment II: comparison of annual, semi-annual and quasi-biennial variations in stratospheric and lower mesospheric water vapour observed from satellites

摘要

In the framework of the second SPARC (Stratosphere-troposphere Processes Andtheir Role in Climate) water vapour assessment (WAVAS-II), the amplitudes andphases of the annual, semi-annual and quasi-biennial variation instratospheric and lower mesospheric water were compared using 30 datasets from 13 different satellite instruments. These comparisons aimed toprovide a comprehensive overview of the typical uncertainties in theobservational database which can be considered in subsequent observationaland modelling studies. For the amplitudes, a good agreement of their latitudeand altitude distribution was found. Quantitatively there were differences inparticular at high latitudes, close to the tropopause and in the lowermesosphere. In these regions, the standard deviation over all data setstypically exceeded 0.2 ppmv for the annual variation and 0.1 ppmv for thesemi-annual and quasi-biennial variation. For the phase, larger differencesbetween the data sets were found in the lower mesosphere. Generally thesmallest phase uncertainties can be observed in regions where the amplitudeof the variability is large. The standard deviations of the phases for alldata sets were typically smaller than a month for the annual and semi-annualvariation and smaller than 5 months for the quasi-biennial variation. Theamplitude and phase differences among the data sets are caused by acombination of factors. In general, differences in the temporal variation ofsystematic errors and in the observational sampling play a dominant role. Inaddition, differences in the vertical resolution of the data, the consideredtime periods and influences of clouds, aerosols as well as non-localthermodynamic equilibrium (NLTE) effects cause differences between theindividual data sets.