Water vapor isotopologue retrievals from high-resolution GOSAT shortwave infrared spectra

摘要

Remote sensing of the isotopic composition of water vapor can providevaluable information on the hydrological cycle. Here, we demonstrate thefeasibility of retrievals of the relative abundance of HDO (the HDO/H₂Oratio) from the Japanese GOSAT satellite. For this purpose, we use highspectral resolution nadir radiances around 6400 cm−1(1.56 μm) to retrieve vertical column amounts of H₂O and HDO.Retrievals of H₂O correlate well with ECMWF (European Centre forMedium-Range Weather Forecasts) integrated profiles (r2 = 0.96). Typicalprecision errors in the retrieved column-averaged deuterium depletion(δD) are 20–40‰. We compare δD against a TCCON(Total Carbon Column Observing Network) ground-based station in Lamont,Oklahoma. Using retrievals in very dry areas over Antarctica, we detect asmall systematic offset in retrieved H₂O and HDO column amounts and takethis into account for a bias correction of δD. Monthly averages ofδD in the June 2009 to September 2011 time frame are well correlatedwith TCCON (r2 = 0.79) and exhibit a slope of 0.98 (1.23 if not biascorrected). We also compare seasonal averages on the global scale withresults from the SCIAMACHY instrument in the 2003–2005 time frame. Despitethe lack of temporal overlap, seasonal averages in general agree well, withspatial correlations (r2) ranging from 0.62 in September through Novemberto 0.83 in June through August. However, we observe higher variability inGOSAT δD, indicated by fitted slopes between 1.2 and 1.46. Thediscrepancies are likely related to differences in vertical sensitivities butwarrant further validation of both GOSAT and SCIAMACHY and an extension ofthe validation dataset.