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Isotope modeling

       The atmospheric general circulation model (AGCM) ECHAM5 is the fifth generation of an AGCM developed at the Max-Planck-Institute in Hamburg (Germany). It was thoroughly tested under present-day conditions (e.g., Roeckner et al., 2003, 2006) and used for the last Intergovernmental Panel on Climate Change assessment (Randall et al., 2007). Regarding the hydrological cycle in the ECHAM5 model, both simulated precipitation amount as well as integrated atmospheric water vapour agree in general well with the observational values over different land surfaces, except for a 10-15% overestimation in the Tropics (Hagemann et al., 2006). 

       For the explicit simulation of HDO and H218O, the ECHAM5 model was enhanced by including a water isotope module in the model’s hydrological cycle (this model version is named ECHAM5-wiso hereafter), following the work of Joussaume et al. (1984), Jouzel et al. (1987), Hoffmann et al. (1998) and Werner et al. (2002). ECHAM5-wiso computes in detail isotopic changes within the entire hydrological cycle, from ocean evaporation through cloud condensation and precipitation (rain- and snowfall) to surface water runoff. First present-day climate control simulations reveal that ECHAM5-wiso results are in good agreement with available observations of water isotopes in precipitation and vapour, as measured at different stations of the Global Network of Isotopes in Precipitation (GNIP, IAEA/WMO, 2006), both on an annual as well as on a seasonal time scale. Due to recent advances in computing power and numerical algorithms, the ECHAM5-wiso model can be run with a very fine spatial grid size of down to 75x75 km on a global scale. For Europe and many other regions, the isotope simulation results clearly benefit from such an increased horizontal and vertical model resolution (Werner et al., 2011).

       To enable a reliable comparison of IGCM modelling results and ground-based as well as satellite measurements of HDO in the atmosphere, we propose for the WSibIso project to run ECHAM5-wiso with very high spatial resolution in a nudged simulation mode for the period 1990-present. In this nudged simulation mode, the model’s state of the atmosphere is constantly relaxed towards the prescribed atmospheric reference state, as defined by the ERA-Interim observational data set (Dee et al., 2011). This ensures that synoptic-scale features within the ECHAM5-wiso simulation follow the observed atmospheric circulation, and the simulated isotopic composition of rainfall for a specific calendar period can be directly compared to the ground-based FTIR measurements or satellite data from the same time period. In addition, the simulation, covering the last two decades, will allow assessing possible changes of Western Siberia’s hydrological cycle and its isotopic composition due to the recent increase in atmospheric greenhouse gases.

       Simulation analyses will focus on the simulated atmospheric HDO signal over Western Siberia region and an in-depth model-data comparison with respect to our scientific objectives in noting that such comparisons will be limited to cloud-free days. Data-Data comparison will focus on the representativeness of HDO at Kourovka for a larger Western Siberia region (FTIR vs. GOSAT or other satellite).

       All proposed ECHAM5-wiso simulations and analyses are computationally very demanding and will be performed on a NEC-SX8 supercomputer, operated by AWI, or supercomputer facilities of the German Climate Computing Centre (DKRZ).

       Water stable isotopes have been recently implemented in the up to date version of the GCM developed at Laboratoire de Météorologie Dynamique, the LMDZ4 model (Hourdin et al., 2006). It is the atmospheric component of the Institut Pierre Simon Laplace (IPSL) ocean-land-atmosphere coupled model (Marti et al., 2005) that participated in CMIP3 (Meehl et al. 2007). The complete LMDZ model solves the primitive equations using finite differences on a 3D eulerian grid. In a standard configuration, 19 sigma-pressure layers are used to discretize the vertical axis. They correspond to a resolution of about 300-500m in the planetary boundary layer (first level at 70 m height) and to a resolution of about 2 km at the tropopause. Its dynamical and physical packages have completely changed since the pioneering work of Joussaume  et al. (1984) who implemented water isotopes in an early version of the LMD GCM.

        An interesting particularity of this GCM is the possibility to use stretched grids (Hourdin et al., 2006), allowing studies at both global and regional  scales (e.g. Krinner et al., 1997). The simulations of water stable isotopes by LMDZ-iso have been evaluated  at different time scales, e.g. .for the present-day isotopic spatial and seasonal distribution and for the isotopic variability at time scales ranging from synoptic to inter-annual. This includes a simulation forced by monthly observed sea surface temperatures (SST) from 1979 to 2007, and in order to evaluate the isotopic simulation in a more rigorous way, a simulation over the same period with the large-scale atmospheric dynamics nudged by meteorological reanalyses (Risi et al., 2010). Since a particular effort has been invested in the representation  of the droplet reevaporation in the model (Bony et al., 2008), a particular attention was given on evaluating the equilibrium between droplets and water vapor using simultaneous vapor and  precipitation data available at some stations (Risi et al., 2010). These authors also pay a lot of attention to evaluating the d-excess, which is sensitive to kinetic fractionation notably during rain reevaporation. Finally, they evaluated the isotopic distribution for two past climates for which isotopic data are available: Last Glacial Maximum (21 000 years ago, 21 ka) and Mid-Holocene (6 ka).

       In WSibIso, we will use LMDZ-iso with a zoom centred around Western Siberia in a nudged mode for present-day conditions. The focus will be on the comparison with surface data (water vapour and precipitation) and on vertical distributions as compared with satellite derived profiles. One further specificity of LMDz-iso is its ability to track the origin of air masses which is important for interpreting isotopic distributions in terms of climatic parameters. This isotopic model will also run under warmer climatic conditions.