Studies on Continental Hydrological Cycle Using Water Isotope Global Circulation Model

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A Water Isotope Global Circulation Model (WI-GCM) is developed based on CCSR/NIES Atmospheric General Circulation Model (AGCM) including global circulation of oxygen and hydrogen isotope with revised treatment of mixed-phase cloud, evaporation of precipitating drop, and evaporation from ocean. The simulated isotope ratio is found to be consistent with the results of GNIP observations. Further, thenumerical scheme of transport of water in the atmosphere is replaced by newly developed grid-base advection scheme with flux-form semi-Lagrangean treatment. It is found that this replacement of transport scheme improves not only the hydrological cycle of polar region but also the simulation of isotope ratio.Comparing the model results with observed isotope ratio in precipitation in Tibet, Siberia, and other regions, some utility of isotope for the study of mechanisms and variability of the hydrological cycle. In Tibet, an importance of water vapor which is originated from evaporation of precipitating drop from convective cloud is suggested. In Siberia, interannual variation of recycling process between land and atmosphere is suggested by observed interannual variation of isotope. In Antarctica, an interannualvariation of isotope ratio related with the interannual change of transport route connected with ENSO variation is identified.Comparing the observed water isotope ratio in the soil and plant with model results it is suggested that the improved treatment of soil water and fractionation by evaporation from land surface is important future study.It is concluded that we have succeeded in building a useful tool for the quantitative studies on hydrological cycle by using water isotope.