Evaluation of WACCM simulations of the upper troposphere and lower stratosphere using data from the Stratosphere-Troposphere Analyses of Regional Transport 2008 experiment (START08)

Data from the Stratosphere-Troposphere Analyses of Regional Transport (START08) field campaign conducted April-June 2008 are used to evaluate trace gas simulations by the Whole-Atmosphere Community Climate Model version 3 (WACCM3). START08 sampled a variety of meteorological conditions over North America. To allow direct comparison with the in situ observations, in this study WACCM dynamics are nudged toward the Goddard Earth Observing System model version 5.1 (GEOS-5.1) analyses. Results show good qualitative agreement overall, but quantitative differences exist, primarily during intrusion events and near the subtropical tropopause break. The temporal evolution of ozone and carbon monoxide on potential temperature surfaces during several tropospheric intrusion events suggest that isentropic transport into the stratosphere is resolved but is mixed too quickly due to model diffusion. Ozone-carbon monoxide and ozone-water vapor relationships are used to assess the ability of the model to reproduce the chemical transition observed in the upper troposphere/lower stratosphere (UT/LS). The simulated mixing branches are more linear than those observed, but the thickness of the transition relative to the thermal tropopause agrees well. This suggests that model diffusion may play a larger role in determining the chemical composition in the UT/LS, as opposed to vertical resolution. Mean vertical profiles of selected trace gases with various lifetimes are also presented. Longer-lived species show little variability in the model troposphere, but agree well in the lower stratosphere. Other species show differences that are specific to their respective source distributions and chemistry.