A test of sensitivity to convective transport in a global atmospheric CO₂ simulation

Two approximations to convective transport have been implemented in an offline chemistry transport model (CTM) to explore the impact on calculated atmospheric CO₂ distributions. Global CO₂ in the year 2000 is simulated using the CTM driven by assimilated meteorological fields from the NASA’s Goddard Earth Observation System Data Assimilation System, Version 4 (GEOS-4). The model simulates atmospheric CO₂ by adopting the same CO₂ emission inventory and dynamical modules as described in Kawa et al. (convective transport scheme denoted as Conv1). Conv1 approximates the convective transport by using the bulk convective mass fluxes to redistribute trace gases. The alternate approximation, Conv2, partitions fluxes into updraft and downdraft, as well as into entrainment and detrainment, and has potential to yield a more realistic simulation of vertical redistribution through deep convection. Replacing Conv1 by Conv2 results in an overestimate of CO₂ over biospheric sink regions. The largest discrepancies result in a CO₂ difference of about 7.8 ppm in the July NH boreal forest, which is about 30% of the CO₂ seasonality for that area. These differences are compared to those produced by emission scenario variations constrained by the framework of Intergovernmental Panel on Climate Change (IPCC) to account for possible land use change and residual terrestrial CO₂ sink. It is shown that the overestimated CO₂ driven by Conv2 can be offset by introducing these supplemental emissions.