Evaluation of near-tropopause ozone distributions in the Global Modeling Initiative combined stratosphere/troposphere model with ozonesonde data

摘要

The NASA Global Modeling Initiative has developed a combinedstratosphere/troposphere chemistry and transport model which fullyrepresents the processes governing atmospheric composition near thetropopause. We evaluate model ozone distributions near the tropopause, usingtwo high vertical resolution monthly mean ozone profile climatologiesconstructed with ozonesonde data, one by averaging on pressure levels andthe other relative to the thermal tropopause. At the tropopause, model ozone is high-biased inthe SH tropics and NH midlatitudes by ~45% in a 4°latitude ×5° longitude model simulation. Doubling the resolution to2°×2.5° increases the NH high bias to ~60%,and reduces the tropical bias to ~30%, apparently due todecreased horizontal transport between the tropics and extratropics in thehigher-resolution simulation. These ozone biases do not appearto be due to an overly vigorous residual circulation, insufficient convection, or excessivestratosphere/troposphere exchange, and so may be due to insufficientvertical resolution or excessive vertical diffusion near the tropopause. Inthe upper troposphere and lower stratosphere, model/measurementintercomparisons are strongly affected by the averaging technique.Compared to the pressure-averaged climatology, NH andtropical mean model lower stratospheric biases are >20%. In the uppertroposphere, the 2°×2.5° simulation shows mean high biases of~20% and ~35% during April in the tropics and NHmidlatitudes, respectively. This apparently good model/measurement agreement degrades whenrelative-to-tropopause averages are considered, with upper troposphere highbiases of ~30% and 70% in the tropics and NH midlatitudes. This occursbecause relative-to-tropopause averaging better preserves the largercross-tropopause O₃ gradients which are seen in the daily sonde data,but not in daily model profiles. Relative-to-tropopause averages thereforemore accurately reveal model/measurement discrepancies.The relative annual cycle of ozone near thetropopause is reproduced very well in the model Northern Hemispheremidlatitudes. In the tropics, the model amplitude of the near-tropopauseannual cycle is weak. This is likely due to the annual amplitude of meanvertical upwelling near the tropopause, which analysis suggests is ~30% weaker than in the real atmosphere.