Resolving the Stratospheric Water Vapour Entry Puzzle using a Combined Model-Measurement Approach
Hegglin, Michaela1; Plummer, David2; Scinocca, John2; Shepherd, Ted1; Reader, Cathy2; Anderson, John3; Froidevaux, Lucien4; Funke, Bernd5; Lumpe, Jerry6; Remsberg, Ellis7; Rozanov, Alexei8; Urban, Joachim9; von Clarmann, Thomas10; Walker, Kaley11; Wang, Ray12
1University of Reading, UNITED KINGDOM; 2Environment Canada, CANADA; 3Hampton University, UNITED STATES; 4JPL, UNITED STATES; 5CSIC, SPAIN; 6Computational Physics Inc, UNITED STATES; 7NASA Langley Research Center, UNITED STATES; 8University of Bremen, GERMANY; 9Chalmers University, SWEDEN; 10KIT, GERMANY; 11University of Toronto, CANADA; 12Georgia Institute of Technology, UNITED STATES

Water vapour is the most important natural greenhouse gas in the atmosphere and provides a positive feedback to the climate forcing from CO2. The greenhouse effect from water vapour is strongest in the upper troposphere and lower stratosphere (UTLS), where the lowest temperatures are found and where strong gradients in concentration across the tropopause exist. Water vapour is also a key constituent in atmospheric chemistry. It is the source of the cleaning agent of the atmosphere, hydroxyl (OH), which controls the lifetime of shorter-lived pollutants, stratospheric and tropospheric ozone, and other long-lived greenhouse gases such as methane. Despite the importance of water vapour to chemistry and the radiative balance of the atmosphere, its observed long-term changes are not well understood mostly due to inadequate quality of the available observations. Comparisons with long-term changes in tropical temperatures that are known to largely control stratospheric water vapour entry values furthermore are inconsistent leading to an unresolved puzzle. We here use a new approach combining water vapour observations from different limb-viewing satellite instruments and the information on modelled water vapour fields from a nudged chemistry-climate model in order to produce a long-term time series (or essential climate variable). This time-series then is evaluated in order to infer and explain observed trends in stratospheric water vapour. We discuss the results in light of the recent findings on measurement accuracy from the SPARC Data Initiative, the first comprehensive multi-instrument comparison of constituent measurements of limb-viewing satellite sounders, point out weaknesses and strengths of the data sets, and future measurement needs for monitoring this key climate agent.