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The Upper Troposphere / Lower Stratosphere (UTLS) is a region difficult to model but crucial as constraining the stratosphere and where occur exchanges of ozone and water vapour between the two layers. Thus, to show the importance of tropospheric chemistry upon the mean atmosphere, two experiments were performed with MOCAGE forced by ARPEGE analyses (off-line mode) : one uses only a stratospheric chemistry scheme (REPROBUS) and was used as reference while the second experiment includes both tropospheric and stratospheric chemistry (RACM + REPROBUS scheme). Each simulation was run over a few days in January and in July. Results upon ozone (Figure 5) showed an increase of ozone in the upper and middle stratosphere, wheres a decrease is found in the tropical UTLS in association with convection wich brings poorer ozone air from the lower troposphere to the UTLS region when tropospheric-stratospheric chemistry is taken into account.
Figure 5 : Snapshots of zonally averaged ozone distributions for January (left colum) and July (right colum) for ozone obtained with the tropospheric-stratospheric chemistry scheme (top), the stratospheric chemistry scheme (middle) and differences in percents (bottom) (yellow and red indicate ozone increase while green and blue indicate decrease)