A set of diagnostics for evaluating chemistry-climate models in the extratropical tropopause region

Three related diagnostics are used to evaluate the representation of chemical transport processes in the extratropical upper troposphere and lower stratosphere (UTLS) by chemistry-transport and chemistry-climate models. The diagnostics are based on in situ observations of ozone, carbon monoxide, water vapor profiles (obtained on board the NASA ER-2 research aircraft, near 65°N and during 1997), and their interrelationships in the UTLS. The first diagnostic compares the observed and modeled UTLS trace gas profiles in a relative altitude coordinate. The second one compares the observed and modeled UTLS tracer relationships. The third one compares the observed and modeled thickness of the tropopause transition layer. Together, they characterize the model's ability to reproduce the observed chemical distribution in the UTLS region and chemical transition across the extratropical tropopause. These are key indicators of whether the contributions of dynamics and chemistry to this region are correctly represented in the models. These diagnostics are used to evaluate the performance of an NCAR chemistry-transport model (CTM), MOZART-3, and a chemistry-climate model (CCM), WACCM3. Results from four model runs with different meteorological fields and grid resolution are examined. Overall, the NCAR models show qualitative agreement with the observations in the location of the chemical transition across the extratropical tropopause. Quantitatively, there are significant differences between the modeled and the observed chemical distributions. Both the meteorological field and grid resolutions are contributing factors to the differences.