Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2018-137
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
14 Feb 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Global IWV trends and variability in atmospheric reanalyses and GPS observations
Ana C. Parracho1,2, Olivier Bock1, and Sophie Bastin2 1IGN LAREG, Univ ersité Paris Diderot, Sorbonne Paris Cité, Paris, 75013, France
2LATMOS/IPSL, UVSQ, Université Paris-Saclay, Sorbonne Universités, UPMC, Univ., Paris 06, CNRS, Guyancourt, France
Abstract. Water vapour plays a key role in the climate system. However, its short residence time in the atmosphere and its high variability in space and time make it challenging when it comes to study trends and variability. There are several sources of water vapour data. In this work we use Integrated Water Vapour (IWV) estimated from GPS observations and atmospheric reanalyses. Monthly and seasonal means, interannual variability, and linear trends are analysed and compared for the period between 1995 and 2010. A general good agreement is found but this study highlights issues in both GPS and reanalysis data sets. In GPS, gaps and inhomogeneities in the time series are evidenced, which affect mainly variability and trend estimation. In ERA-Interim, too strong trends in certain regions (e.g. drying over northern Africa and Australia, and moistening over northern South America) were found. Representativeness differences in coastal areas and regions of complex topography (mountain ranges, islands) are also evidenced as limitations to the intercomparison of the point observations and reanalysis data. A general good agreement is found for the means and variabilities, with the exception of a few stations where representativeness issues are suspected. Monthly IWV trends are also found to be in good sign agreement, with the exception of a handful of stations where, in addition to representativeness errors, there might be inhomogeneities in the GPS time series. Seasonal trends are found to be different and more intense than monthly trends, which emphasizes the influence of atmospheric circulation on IWV trends. In order to assess strong trends over regions lacking GPS stations, a second reanalysis, MERRA-2, is introduced. The period of analysis is extended to 1980–2016 (the longest period the reanalyses have in common) and differences with the shorter period are found. This exemplifies how much IWV trends are dependent on the time period at study and must be interpreted carefully. Temperature trends are also computed for both reanalyses. The Clausius-Clapeyron scaling ratio is found to not be a good humidity proxy at seasonal and regional scales. Regions over northern Africa and Australia, where ERA-Interim and MERRA-2 disagree, are investigated further. Dynamics at these regions is assessed by analyzing the wind fields at 925 hPa and is shown to be tightly linked with the trends and variability in IWV.

Citation: Parracho, A. C., Bock, O., and Bastin, S.: Global IWV trends and variability in atmospheric reanalyses and GPS observations, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-137, in review, 2018.
Ana C. Parracho et al.
Ana C. Parracho et al.
Ana C. Parracho et al.

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Short summary
Integrated water vapour from GPS observations and atmospheric reanalyses were compared for 1995–2010. Means, variability and trend signs were in general good agreement at most GPS stations. Stations with poor agreement were investigated further. Representativeness issues and inhomogeneities in GPS were evidenced. Reanalyses were also intercompared for an extended period, and a focus on North Africa and Australia highlighted the impact of dynamics on water vapour trends.
Integrated water vapour from GPS observations and atmospheric reanalyses were compared for...
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