A posteriori calculation of δ 18O and δD in atmospheric water vapour from ground-based near-infrared FTIR retrievals of H 216O, H 218O, and HD 16O
This paper investigates the scientific value of retrieving H
218O and HDO columns in addition to H
216O columns from high-resolution ground-based near-infrared spectra. We present a set of refined H
218O, and HDO spectral windows. The retrieved H
218O, and HDO columns are used for an a posteriori calculation of columnar δD and δ
18O. We estimate the uncertainties for the so-calculated columnar δD and δ
18O values. These estimations include uncertainties due to the measurement noise, errors in the a priori data, and uncertainties in spectroscopic parameters. Time series of δ
18O obtained from ground-based FTIR (Fourier transform infrared) spectra are presented for the first time.
For our study we use a full physics isotopic general circulation model (ECHAM5-wiso). We show that the full physics simulation of HDO and H 218O can already be reasonably predicted from the H 216O columns by a simple linear regression model (scatter values between full physics and linear regression simulations are 35 and 4‰ for HDO and H 218O, respectively). We document that the columnar δD and δ 18O values as calculated a posteriori from the retrievals of H 216O, H 218O, and HDO show a better agreement with the ECHAM5-wiso simulation than the δD and δ 18O values as calculated from the H 216O retrievals and the simple linear regression model. This suggests that the H 218O and HDO column retrievals add complementary information to the H 216O retrievals. However, these data have to be used carefully, because of the different vertical sensitivity of the H 216O, H 218O, and HDO columnar retrievals. Furthermore, we have to note that the retrievals use reanalysis humidity profiles as a priori input and the results are thus not independent of the reanalysis data.