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

Abstract

In the framework of the second SPARC (Stratosphere-troposphere Processes And their Role in Climate) water vapour assessment (WAVAS-II), the amplitudes and phases of the annual, semi-annual and quasi-biennial variation in stratospheric and lower mesospheric water were compared using 30 data sets from 13 different satellite instruments. These comparisons aimed to provide a comprehensive overview of the typical uncertainties in the observational database which can be considered in subsequent observational and modelling studies. For the amplitudes, a good agreement of their latitude and altitude distribution was found. Quantitatively there were differences in particular at high latitudes, close to the tropopause and in the lower mesosphere. In these regions, the standard deviation over all data sets typically exceeded 0.2 ppmv for the annual variation and 0.1 ppmv for the semi-annual and quasi-biennial variation. For the phase, larger differences between the data sets were found in the lower mesosphere. Generally the smallest phase uncertainties can be observed in regions where the amplitude of the variability is large. The standard deviations of the phases for all data sets were typically smaller than a month for the annual and semi-annual variation and smaller than Published by Copernicus Publications on behalf of the European Geosciences Union.

An “island” in the stratosphere – on the enhanced annual variation of water vapour in the middle and upper stratosphere in the southern tropics and subtropics

“…The peak amplitude is 0.51 ppmv. This is somewhat lower than in the MI-PAS observations, but still within the range of the different observational data sets compared in the second SPARC water vapour assessment (Lossow et al, 2017). The amplitude agreement at 15 • S is within ±0.05 ppmv right below the peak altitude down to about 6 hPa.…”

“…The MIPAS data exhibit an overall peak amplitude of 0.58 ppmv. The data sets compared within the second SPARC water vapour assessment (WAVAS-II) indicate on average a peak amplitude of almost 0.65 ppmv and a corresponding standard deviation of 0.1 ppmv (not shown here, Lossow et al, 2017). The white contour line in Fig.…”

“…Most prominently the annual minimum exhibits very low ratios in 2006, 2008 and 2011, while in 2007 and 2009 the ratios are clearly higher. This variability is predominantly related to the QBO, which has a local maximum in this region (Randel et al, 1998;Lossow et al, 2017).…”

“…Also, the location is rather dependent on the actual approach chosen to derive the characteristics of the annual variation (see Sect. 5.3 of Lossow et al, 2017).…”

“…All geometric altitudes noted in this paper are based on this quantity. The characteristics of the annual variation have been obtained by means of regression, employing a regression model containing an offset, a linear term, the semi-annual and annual variation as well a QBO (quasi-biennial oscillation) term (for more details please see Lossow et al, 2017). The largest stratospheric annual variability is found in the polar regions and close to the tropopause.…”