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

Abstract. This paper investigates the scientific value of retrieving H₂¹⁸O and HDO columns in addition to H₂¹⁶O columns from high-resolution ground-based near-infrared spectra. We present a set of refined H₂¹⁶O, H₂¹⁸O, and HDO spectral windows. The retrieved H₂¹⁶O, H₂¹⁸O, and HDO columns are used for an a posteriori calculation of columnar δD and δ¹⁸O. We estimate the uncertainties for the so-calculated columnar δD and δ¹⁸O values. These estimations include uncertainties due to the measurement noise, errors in the a priori data, and uncertainties in spectroscopic parameters. Time series of δ¹⁸O 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 H₂¹⁸O can already be reasonably predicted from the H₂¹⁶O columns by a simple linear regression model (scatter values between full physics and linear regression simulations are 35 and 4‰ for HDO and H₂¹⁸O, respectively). We document that the columnar δD and δ¹⁸O values as calculated a posteriori from the retrievals of H₂¹⁶O, H₂¹⁸O, and HDO show a better agreement with the ECHAM5-wiso simulation than the δD and δ¹⁸O values as calculated from the H₂¹⁶O retrievals and the simple linear regression model. This suggests that the H₂¹⁸O and HDO column retrievals add complementary information to the H₂¹⁶O retrievals. However, these data have to be used carefully, because of the different vertical sensitivity of the H₂¹⁶O, H₂¹⁸O, 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.