Modeling Water Isotopes Using a Global Non‐Hydrostatic Model With an Explicit Convection: Comparison With Gridded Data Sets and Site Observations

<jats:title>Abstract</jats:title><jats:p>In this study, we developed a global cloud system‐resolving model (GCSRM) incorporating stable water isotopes (NICAM‐WISO). Using a single‐moment cloud microphysics scheme, we applied the new model to conduct a current climate simulation at a horizontal resolution of 56 km. NICAM‐WISO simulated the seasonal means of the atmospheric hydrological cycle, as well as water isotopic ratios of precipitation and vapor. The model captured the general features of precipitation isotope effects and its spatial correlations were comparable to those of other isotope‐incorporated global atmospheric models. The model showed better spatial correlation between simulated and observed values for a fine‐horizontal‐resolution (14 km) simulation compared to coarse‐horizontal‐resolution (56 km) simulation. However, the model had isotopic biases in tropical mid‐troposphere, ocean, and cold continental regions. A comparison of stable water isotopes between the simulation and observations offered clues for improving the model. For example, in the tropical mid‐troposphere, we found a negative bias in the mixing ratio and isotopic ratio of water vapor. Our analysis using satellite retrievals revealed that these underestimations were caused by weak mixing with the boundary layer vapor and low raindrop evaporation with a small evaporation fraction. The underestimations indicated weak shallow convective mixing in the model, inducing negative bias in the mixing ratio and isotopic ratio of the mid‐tropospheric vapor. These biases were also seen in the fine horizontal‐resolution simulation. Furthermore, we conducted several km‐scale atmospheric isotope circulation simulations using NICAM‐WISO. We expect that global‐scale fine‐horizontal‐resolution simulations using isotope‐incorporated GCSRMs will improve our understanding of the atmospheric hydrological cycle.</jats:p>