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Published April 8, 2013 | Supplemental Material + Published
Journal Article Open

Improved water vapour spectroscopy in the 4174-4300 cm⁻¹ region and its impact on SCIAMACHY HDO/H₂O measurements

Abstract

The relative abundance of the heavy water isotopologue HDO provides a deeper insight into the atmospheric hydrological cycle. The SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) allows for global retrievals of the ratio HDO/HO in the 2.3 micron wavelength range. However, the spectroscopy of water lines in this region remains a large source of uncertainty for these retrievals. We therefore evaluate and improve the water spectroscopy in the range 4174–4300 cm⁻¹ and test if this reduces systematic uncertainties in the SCIAMACHY retrievals of HDO/H₂O. We use a laboratory spectrum of water vapour to fit line intensity, air broadening and wavelength shift parameters. The improved spectroscopy is tested on a series of ground-based high resolution FTS spectra as well as on SCIAMACHY retrievals of H2O and the ratio HDO/H₂O. We find that the improved spectroscopy leads to lower residuals in the FTS spectra compared to HITRAN 2008 and Jenouvrier et al. (2007) spectroscopy, and the retrievals become more robust against changes in the retrieval window. For both the FTS and SCIAMACHY measurements, the retrieved total H₂O columns decrease by 2–4% and we find a negative shift of the HDO/H₂O ratio, which for SCIAMACHY is partly compensated by changes in the retrieval setup and calibration software. The updated SCIAMACHY HDO/H₂O product shows somewhat steeper latitudinal and temporal gradients and a steeper Rayleigh distillation curve, strengthening previous conclusions that current isotope-enabled general circulation models underestimate the variability in the near-surface HDO/H₂O ratio.

Additional Information

© Author(s) 2013. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 02 Sep 2012 – Discussion started: 29 Nov 2012 – Revised: 22 Feb 2013 – Accepted: 13 Mar 2013 – Published: 08 Apr 2013. This researched was funded by the Netherlands Space Office as part of the User Support Programme Space Research under project GO-AO/16. AB is funded through the Emmy-Noether programme of Deutsche Forschungsgemeinschaft (DFG) through grant BU2599/1-1 (RemoteC). SF is funded by the Belgian Federal Science Policy Office, the Fonds National de la Recherche Scientifique (FNRS, Belgium) and the European Space Agency (ESA-Prodex program). US funding for TCCON comes from NASA's Terrestrial Ecology Program, grant number NNX11AG01G, the Orbiting Carbon Observatory Program, the Atmospheric CO2 Observations from Space (ACOS) Program and the DOE/ARM Program. The Darwin TCCON site was built at Caltech with funding from the OCO project, and is operated by the University of Wollongong, with travel funds for maintenance and equipment costs funded by the OCO-2 project. We acknowledge funding to support Darwin from the Australian Research Council, Projects LE0668470, DP0879468, DP110103118 and LP0562346. We further acknowledge David Griffith and Ronald Macatangay for providing the Darwin TCCON spectra and data. We acknowledge the Alfred Wegener Institute for Polar and Marine research (AWI) for providing the measurement infrastructure at Ny Ålesund and Cor Becker and his team from the Meteorological Service of Suriname for their support for the measurements at Paramaribo (Suriname). ECMWF ERA-Interim data used in this study have been obtained from the ECMWF data server. We thank Richard van Hees for his work on the nadc tools software package, Pieter van der Meer for his efforts to keep the SRON cluster running and Paul Tol for his help with converting HITRAN formats. We thank the anonymous referees for their comments that improved this paper. Edited by: K. Strong.

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