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. Within the framework of the second SPARC (Stratosphere-troposphere ProcessesAnd their Role in Climate) water vapour assessment (WAVAS-II), weevaluated five data sets of δD(H2O) obtained fromobservations by Odin/SMR (Sub-Millimetre Radiometer), Envisat/MIPAS(Environmental Satellite/Michelson Interferometer for Passive AtmosphericSounding), and SCISAT/ACE-FTS (Science Satellite/Atmospheric ChemistryExperiment – Fourier Transform Spectrometer) using profile-to-profile andclimatological comparisons. These comparisons aimed to provide acomprehensive overview of typical uncertainties in the observational databasethat could be considered in the future in observational and modelling studies.Our primary focus is on stratospheric altitudes, but results for the uppertroposphere and lower mesosphere are also shown. There are clear quantitativedifferences in the measurements of the isotopic ratio, mainly with regard tocomparisons between the SMR data set and both the MIPAS and ACE-FTS datasets. In the lower stratosphere, the SMR data set shows a higher depletion in δD than the MIPAS and ACE-FTS data sets. The differencesmaximise close to 50 hPa and exceed 200 ‰. With increasing altitude,the biases decrease. Above 4 hPa, the SMR data set shows a lower δD depletion than the MIPAS data sets, occasionally exceeding100 ‰. Overall, the δD biases of the SMR data set aredriven by HDO biases in the lower stratosphere and by H2O biases inthe upper stratosphere and lower mesosphere. In between, in the middlestratosphere, the biases in δD are the result of deviations inboth HDO and H2O . These biases are attributed to issues with thecalibration, in particular in terms of the sideband filtering, anduncertainties in spectroscopic parameters. The MIPAS and ACE-FTS data setsagree rather well between about 100 and 10 hPa. The MIPAS data sets showless depletion below approximately 15 hPa (up to about 30 ‰), due todifferences in both HDO and H2O . Higher up this behaviour is reversed,and towards the upper stratosphere the biases increase. This is driven byincreasing biases in H2O , and on occasion the differences in δD exceed 80 ‰. Below 100 hPa, the differences betweenthe MIPAS and ACE-FTS data sets are even larger. In the climatologicalcomparisons, the MIPAS data sets continue to show less depletion in δD than the ACE-FTS data sets below 15 hPa during all seasons,with some variations in magnitude. The differences between the MIPAS andACE-FTS data have multiple causes, such as differences in the temporal andspatial sampling (except for the profile-to-profile comparisons), cloudinfluence, vertical resolution, and the microwindows and spectroscopicdatabase chosen. Differences between data sets from the same instrument aretypically small in the stratosphere. Overall, if the data sets are consideredtogether, the differences in δD among them in key areas ofscientific interest (e.g. tropical and polar lower stratosphere, lowermesosphere, and upper troposphere) are too large to draw robust conclusions onatmospheric processes affecting the water vapour budget and distribution,e.g. the relative importance of different mechanisms transporting watervapour into the stratosphere.

atmospheric chemistry and physics

The SPARC water vapour assessment II: profile-to-profile and climatological comparisons of stratospheric &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;D(H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O) observations from satellite