Structure and processes of the upper troposphere and lower stratosphere, and their sensitivity to changes in atmospheric CO2 concentrations

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The separation of troposphere and stratosphere is traditionally based on criteria of the vertical temperature structure, with a sharp tropopause at an altitude of about 10 km, that bulges steeply upward over the subtropics to an altitude of about 17 km over the tropics. However, other characteristics such as radiative properties or correlations of trace gas concentrations suggest a more gradual transition over a vertical layer extending from the upper troposphere (UT) into the lower stratosphere (LS). The structure of the UT/LS is determined both by tropospheric and stratospheric processes, and may be sensitive to changes in atmospheric carbon dioxide concentrations. Conversely, changes in the UT/LS may have a strong feedback on climate. The proposed project will help to improve our understanding of the processes that control the UT/LS structure in order to provide improved predictions of how this layer changes in a changing climate, and how the changes in this layer may have an impact on climate. To this end, we will make use of space-borne atmospheric observations, as well as of in-situ observations from instruments onboard of high flying aircrafts. The analysis of these observations will help to formulate hypotheses, which will be tested with the help of large-scale numerical models, ranging in complexity from primitive equation models to coupled chemistry/climate models.