ALBERT

Description: Stable water isotopes in the coupled atmosphere–land surface model ECHAM5-JSBACH Geoscientific Model Development Discussions, 5, 3375-3418, 2012 Author(s): B. Haese, M. Werner, and G. Lohmann In this study we present first results of a new model development, ECHAM5-JSBACH-wiso, where we have incorporated the stable water isotopes H 2 18 O and HDO as tracers in the hydrological cycle of the coupled atmosphere–land surface model ECHAM5-JSBACH. The ECHAM5-JSBACH-wiso model was run under present-day climate conditions at two different resolutions (T31L19, T63L31). A comparison between ECHAM5-JSBACH-wiso and ECHAM5-wiso shows that the coupling has a strong impact on the simulated temperature and soil wetness. Caused by these changes of temperature and the hydrological cycle, the δ 18 O in precipitation also shows variations from −4.5‰ up to 4.5‰. One of the clearest anomalies is shown over North-East Asia where, depending on an increase of temperature, the δ 18 O in precipitation increases as well. In order to analyze the sensitivity of the fractionation processes over land, we compare a set of simulations with various implementations of water isotope fractionation processes over the land surface. The simulations allow us to distinguish between no fractionation, fractionation included in the evaporation flux (from bare soil) and also fractionation included in both evaporation and transpiration (from water transport through plants) fluxes. The simulated δ 18 O and δD in precipitation of these setups generally fit well with the observations and the best agreement between observation and simulation is given in the case where no fractionation over land surface is assumed.