Evaluation of a Three-Dimensional Global Model in Simulating Upper-Tropospheric Chemistry

The impact of anthropogenic activity on radiatively active trace species in the tropopause region (Upper Troposphere and Lower stratosphere or UTLS) is still poorly known. The climate sensitivity, however, is particularly high in this region. Many modeling studies address the anthropogenic ozone and methane perturbations in the UTLS, but it is virtually unknown to what extent global chemistry-transport models (CTMs) are capable of adequately simulating such perturbations. Modeling the chemical composition of the UTLS is very difficult, since this region is characterized by strong cross-tropopause concentration gradients, mixing between the stratosphere and the troposphere, and by deep convection. In order to evaluate the performance of our CTM (TM3) in quantifying the effects of anthropogenic activity on UTLS ozone and methane concentrations, we evaluated the model results with the observations of the SONEX campaign, conducted in October and November 1997 in the UTLS over parts of the U.S, Europe and the northern Atlantic Ocean. We compared simulated concentrations of trace gases and radicals, as well as OH and HO₂ loss and production rates, with the observed values. In particular we discuss the role of non-methane hydrocarbons, the influence of convection, and ECMWF water vapor biases. We also discuss the application of the new ECMWF reanalysis data (ERA40). Despite several problems the model reproduced the observed rates reasonably well. On average the modeled ozone production was within 30% of the observed value. For samples not exposed to recent convection and/or lightning the modeled ozone production was within 20% of the observed.