Mid-tropospheric δD observations from IASI/MetOp at high spatial and temporal resolution

In this paper we present a method to retrieve HDO, H2O and δD from IASI radiances spectra. It relies on an existing radiative transfer model (Atmosphit) and optimal estimation inversion scheme, but goes further than our previous work by explicitly considering correlations between the two species. Global (fixed) HDO and H2O a priori profiles together with a covariance matrix were built from daily model simulations of HDO and H2O profiles (the LMDz-iso model is used) over the whole globe and a whole year. The retrieval parameters are described and characterized in terms of errors. We show that IASI is mostly sensitive to δD in the middle troposphere and allows retrieving δD for an integrated 3-6 km column with an error of 38‰ on an individual measurement basis. We examine the performance of the retrieval to capture the temporal (seasonal and short-term) and spatial variations of δD by analyzing one year of measurement at two dedicated sites (Darwin and Izaña) and a latitudinal band from −60° to 60° for a 15 days period in January. The performances are compared with LMDz-iso simulations. We report a general excellent agreement between IASI and the model and demonstrate the capabilities of IASI to reproduce the large scale variations of δD (seasonal cycle and latitudinal gradient) with good accuracy. In particular, we show that there is no systematic significant bias in the retrieved δD values in comparison with the model, and that the retrieved variability is similar to the modeled one although there are certain significant differences depending on the location. Moreover the noticeable differences between IASI and the model reported and briefly examined tend to suggest modeling issues instead of retrieval effects. Finally, we strengthen the unprecedented capabilities of IASI to capture short-term variations in δD, further highlighting the added value of the sounder for monitoring hydrological processes