A posteriori calculation of δ18O and δD in atmospheric water vapour from ground-based near-infrared FTIR retrievals of H216O, H218O, and HD16O

This paper investigates the scientific value of retrieving (H2O)-O-18 and HDO columns in addition to (H2O)-O-16 columns from high-resolution ground-based near-infrared spectra. We present a set of refined (H2O)-O-16, (H2O)-O-18, and HDO spectral windows. The retrieved (H2O)-O-16, (H2O)-O-18, and HDO columns are used for an a posteriori calculation of columnar delta D and delta O-18. We estimate the uncertainties for the so-calculated columnar delta D and delta O-18 values. These estimations include uncertainties due to the measurement noise, errors in the a priori data, and uncertainties in spectroscopic parameters. Time series of delta O-18 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 (H2O)-O-18 can already be reasonably predicted from the (H2O)-O-16 columns by a simple linear regression model (scatter values between full physics and linear regression simulations are 35 and 4 for HDO and (H2O)-O-18, respectively). We document that the columnar delta D and delta O-18 values as calculated a posteriori from the retrievals of (H2O)-O-16, (H2O)-O-18, and HDO show a better agreement with the ECHAM5-wiso simulation than the delta D and delta O-18 values as calculated from the (H2O)-O-16 retrievals and the simple linear regression model. This suggests that the (H2O)-O-18 and HDO column retrievals add complementary information to the (H2O)-O-16 retrievals. However, these data have to be used carefully, because of the different vertical sensitivity of the (H2O)-O-16, (H2O)-O-18, 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.