Simulation of upper tropospheric CO₂ from chemistry and transport models

The California Institute of Technology/Jet Propulsion Laboratory two‐dimensional (2‐D), three‐dimensional (3‐D) GEOS‐Chem, and 3‐D MOZART‐2 chemistry and transport models (CTMs), driven respectively by NCEP2, GEOS‐4, and NCEP1 reanalysis data, have been used to simulate upper tropospheric CO 2 from 2000 to 2004. Model results of CO 2 mixing ratios agree well with monthly mean aircraft observations at altitudes between 8 and 13 km (Matsueda et al., 2002) in the tropics. The upper tropospheric CO 2 seasonal cycle phases are well captured by the CTMs. Model results have smaller seasonal cycle amplitudes in the Southern Hemisphere compared with those in the Northern Hemisphere, which are consistent with the aircraft data. Some discrepancies are evident between the model and aircraft data in the midlatitudes, where models tend to underestimate the amplitude of CO 2 seasonal cycle. Comparison of the simulated vertical profiles of CO 2 between the different models reveals that the convection in the 3‐D models is likely too weak in boreal winter and spring. Model sensitivity studies suggest that convection mass flux is important for the correct simulation of upper tropospheric CO 2 .