Improved Chemical Tracer Simulation by MIROC4.0-based Atmospheric Chemistry-Transport Model (MIROC4-ACTM)

The accuracy of chemical tracer simulations by atmospheric general circulation model (AGCM)-based chemistry-transport models (ACTMs) depends on the quality of AGCM transport properties, even when the meteorology is nudged towards the reanalysis fields. Here we show that significant improvements in tracer distribution are achieved when hybrid vertical coordinate is implemented in MIROC4.0 AGCM, compared to its predecessors AGCM5.7b based on sigma coordinate. Only explicitly resolved gravity waves are propagated into the stratosphere in MIROC4-ACTM. The MIROC4-ACTM produces “age-of-air” up to about 5 years in the tropical upper stratosphere (∼1 hPa) and about 6 years in the polar middle stratosphere (∼10 hPa), in agreement with observational estimates. Comparisons of MIROC4-ACTM simulation with observed sulphur hexafluoride (SF6) in the troposphere also show remarkable improvements over the AGCM57b-ACTM simulation. MIROC4-ACTM is characterized by weaker convective mass flux and thus older age of air in the tropical troposphere, relative to AGCM57b-ACTM. The role of convective transport on tracer simulations is depicted using vertical cross-sections of 222Rn (radon) distributions. Both the ACTM versions show similar results when compared with 222Rn measurements at remote sites. All aspects of tracer transport in MIROC4-ACTM is promising for inverse modelling of greenhouse gases sources and sinks at reduced bias.