Optimal estimation of tropospheric H 2 O and δD with IASI/METOP

We present optimal estimates of tropospheric H2O and D derived from radiances measured by the instrument IASI (Infrared Atmospheric Sounding Interferometer) flown on EUMETSAT's polar orbiter METOP. We document that the IASI spectra allow for retrieving H2O profiles between the surface and the upper troposphere as well as middle tro- pospheric D values. A theoretical error estimation suggests a precision for H2O of better than 35 % in the lower tro- posphere and of better than 15 % in the middle and upper troposphere, respectively, whereby surface emissivity and atmospheric temperature uncertainties are the leading error sources. For the middle tropospheric D values we estimate a precision of 15-20 ‰ with the measurement noise being the dominating error source. The accuracy of the IASI products is estimated to about 20-10 % and 10 ‰ for lower to upper tropospheric H2O and middle tropospheric D, respectively. It is limited by systematic uncertainties in the applied spec- troscopic parameters and the a priori atmospheric tempera- ture profiles. We compare our IASI products to a large num- ber of near coincident radiosonde in-situ and ground-based FTS (Fourier Transform Spectrometer) remote sensing mea- surements. The bias and the scatter between the different H2O and D data sets are consistent with the combined theo- retical uncertainties of the involved measurement techniques.

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