Evaluation of Transort in the Lower Tropical Stratosphere in a Global Chemistry and Transport Model

In order to compare the transport characteristics of an assimilated dataset with those of the parent general circulation model (GCM), a set of off-line transport experiments has been performed. The tracer-estimated age of air inferred from observations of SF6 and CO2 is compared to the values determined from the GCM and the data assimilation system (DAS). Distributions of ozone, total reactive nitrogen, and methane from the two simulations are also compared with observations. These comparisons show that DAS fields produce too rapid ascent in the Tropics and excessive mixing between the Tropics and middle latitudes; these features are much better represented in the GCM. The unrealistic transport produced by the DAS fields may be due to implicit forcing that is required by the assimilation process when there is bias between the GCM forecast and observations that are combined to produce the analyzed fields.