The SPARC water vapour assessment II: profile-to-profile and climatological comparisons of stratospheric δD(H2O) observations from satellite
Authors: Högberg Charlotta , Lossow Stefan , Khosrawi Farahnaz , Bauer Ralf , Walker Kaley A. , Eriksson Patrick , Murtagh Donal P. , Stiller Gabriele P. , Steinwagner Jörg , Zhang Qiong 
Published: February 2019

Abstract. Within the framework of the second SPARC (Stratosphere-troposphere Processes
And their Role in Climate) water vapour assessment (WAVAS-II), we
evaluated five data sets of δD(H2O) obtained from
observations by Odin/SMR (Sub-Millimetre Radiometer), Envisat/MIPAS
(Environmental Satellite/Michelson Interferometer for Passive Atmospheric
Sounding), and SCISAT/ACE-FTS (Science Satellite/Atmospheric Chemistry
Experiment – Fourier Transform Spectrometer) using profile-to-profile and
climatological comparisons. These comparisons aimed to provide a
comprehensive overview of typical uncertainties in the observational database
that could be considered in the future in observational and modelling studies.
Our primary focus is on stratospheric altitudes, but results for the upper
troposphere and lower mesosphere are also shown. There are clear quantitative
differences in the measurements of the isotopic ratio, mainly with regard to
comparisons between the SMR data set and both the MIPAS and ACE-FTS data
sets. In the lower stratosphere, the SMR data set shows a higher depletion in
δD than the MIPAS and ACE-FTS data sets. The differences
maximise 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 exceeding
100 ‰. Overall, the δD biases of the SMR data set are
driven by HDO biases in the lower stratosphere and by H2O biases in
the upper stratosphere and lower mesosphere. In between, in the middle
stratosphere, the biases in δD are the result of deviations in
both HDO and H2O. These biases are attributed to issues with the
calibration, in particular in terms of the sideband filtering, and
uncertainties in spectroscopic parameters. The MIPAS and ACE-FTS data sets
agree rather well between about 100 and 10 hPa. The MIPAS data sets show
less depletion below approximately 15 hPa (up to about 30 ‰), due to
differences in both HDO and H2O. Higher up this behaviour is reversed,
and towards the upper stratosphere the biases increase. This is driven by
increasing biases in H2O, and on occasion the differences in
δD exceed 80 ‰. Below 100 hPa, the differences between
the MIPAS and ACE-FTS data sets are even larger. In the climatological
comparisons, 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 and
ACE-FTS data have multiple causes, such as differences in the temporal and
spatial sampling (except for the profile-to-profile comparisons), cloud
influence, vertical resolution, and the microwindows and spectroscopic
database chosen. Differences between data sets from the same instrument are
typically small in the stratosphere. Overall, if the data sets are considered
together, the differences in δD among them in key areas of
scientific interest (e.g. tropical and polar lower stratosphere, lower
mesosphere, and upper troposphere) are too large to draw robust conclusions on
atmospheric processes affecting the water vapour budget and distribution,
e.g. the relative importance of different mechanisms transporting water
vapour into the stratosphere.