1Spectroscopie de l'Atmosphère, Service de Chimie Quantique et de Photophysique, Université Libre de Bruxelles (U.L.B.), Brussels, Belgium
2Service d'Aéronomie/IPSL, CNRS, Université Pierre et Marie Curie, Paris, France
*now at: Laboratoire d'Aérologie, UMR 5560 CNRS/Université Paul Sabatier, Observatoire de Midi-Pyrénées, Toulouse, France
**Research Associate with the F.N.R.S, Belgium
Abstract. The analysis of the atmospheric isotopologic water vapour composition provides valuable information on many climate, chemical and atmospheric circulation processes. The remote-sensing of the water isotopologues remains a challenge, which is enhanced by the large and fast variations of their spatial distributions. This paper presents for the first time the simultaneous retrieval of global distributions of the main water isotopologues (i.e.~H216O, H218O, HOD) and their ratios. The results are obtained by exploiting the high resolution infrared spectra recorded by the Interferometric Monitor for Greenhouse gases (IMG) instrument, which operated in the nadir geometry onboard the ADEOS satellite between 1996 and 1997. The retrievals are performed on a series of cloud-free radiance measurements in two atmospheric windows (1205–1228 cm–1; 2004–2032 cm–1) using a line-by-line radiative transfer model and an inversion procedure based on the Optimal Estimation Method (OEM). Characterizations in terms of vertical sensitivity and error budget are provided. A relatively high vertical resolution is achieved for H216O (~4–5 km), and we show that the retrieved profiles are in good agreement with local sonde measurements, representative of different latitudes. The retrieved global distributions of H216O, H218O, HOD and their ratios are also found to be consistent with previous experimental studies and models. The ocean-earth difference, the latitudinal and vertical dependence of the water vapour amount and the isotopologic depletion are notably well reproduced. Others trends, possibly related to smaller scales variations in the vertical profiles are also discussed. Despite the difficulties encountered for computing accurately the isotopologic ratios, our results demonstrate the ability of the infrared nadir sounding for monitoring atmospheric isotopologic water vapour distributions on a global scale.