Three-dimensional simulation of stratospheric gravitational separation using the NIES global atmospheric tracer transport model

The process of atmospheric molecules separation depending on their molar masses due to gravity is gravitational separation (GS). Recently, the existence of GS for the major atmospheric components in the upper troposphere and lower stratosphere was confirmed both experimentally using a precise cryogenic sampler and theoretically by 2-dimensional numerical simulations with SOCRATES model. Here we extend this work by performing a more quantitative analysis using ¹²C¹⁶O₂ and ¹³C¹⁶O₂ distributions simulated by the 3-dimensional simulation by the National Institute for Environmental Studies (NIES) transport model with the molecular diffusion parameterization. The modeled value is compared to observations and the zonal mean from 2-dimensional model SOCRATES. In comparison with the SOCRATES simulation, the NIES model has a number of significant advantages: a three-dimensional tracer transport simulation driven by global JCDAS reanalysis and a vertical coordinate with isentropic levels. The model is optimized to run greenhouse gas simulation, as confirmed through various validation and multi-model inter-comparisons. The use of optimized CO₂ fluxes provided realistic tracer distribution and seasonality. The model-to-observation comparison shows that the model with this molecular diffusion parameterization is able to reproduce the mean value and the number of small-scale fluctuations recorded by high-precision cryogenic balloon-borne observations in the lower stratosphere. We investigated the GS by following the formulation in the SOCRATES model. Under this framework, the seasonal variations of pressure and temperature gradients are the main contributors that drive molecular diffusion and are thus the cause of variations in the GS. We found a strong relationship between the modeled GS and age of air, which is the main indicator of circulation in the stratosphere. However, in contrast to the age of air, the GS has a lower sensitivity to seasonal variability, which is a significant issue in the study of the atmospheric circulation. Thus, the modeled GS characteristics complement age of air information to give a more comprehensive study of the upper troposphere and lower stratosphere structure changes.