Three-dimensional simulation of stratospheric gravitational separation using the NIES global atmospheric tracer transport model
A three-dimensional simulation of gravitational separation, defined as the process of atmospheric molecule separation under gravity according to their molar masses, is performed for the first time in the upper troposphere and lower stratosphere. We analyze distributions of two isotopes with a small difference in molecular mass (13C16O2 (Mi=45) and 12C16O2 (Mi=44)) simulated by the National Institute for Environmental Studies (NIES) chemical transport model (TM) with a parameterization of molecular diffusion. The NIES model employs global reanalysis and an isentropic vertical coordinate and uses optimized CO2 fluxes. The applicability of the NIES TM to the modeling of gravitational separation is demonstrated by a comparison with measurements recorded by high-precision cryogenic balloon-borne samplers in the lower stratosphere. We investigate the processes affecting the seasonality of gravitational separation and examine the age of air derived from the tracer distributions modeled by the NIES TM. We find a strong relationship between age of air and gravitational separation for the main climatic zones. The advantages and limitations of using age of air and gravitational separation as indicators of the variability in the stratosphere circulation are discussed.