Trace gas transport on global and regional scales in the light of a new model system: mixing and mixing lines at the polar front
Final Report Abstract
Although a large fraction of tropospheric ozone is produced photochemically, significant amounts of ozone have stratospheric origin and are transported to the troposphere along deep tropopause folds in cyclones. This transport of air masses from the stratosphere to the troposphere (STT) can lead to peaked ozone concentrations at ground level and hereby influence the long-term trend of tropospheric ozone. Because of their coarse resolution, global models are not able to simulate these STT-processes in detail. Therefore we use the global and regional atmospheric chemistry model system MECO(n), which couples the limited-area atmospheric chemistry and climate model COSMO/MESSy to the global model ECHAM5/MESSy for Atmospheric Chemistry (EMAC). Using similar process parametrisations, the system allows for very consistent, simultaneous simulations in different spatial resolutions. As a part of this project a new submodel has been developed, enabling the release of artificial, passive tracers dependent on certain variables. Initializing an artificial stratospheric tracer that way allows distinguishing between ozone with stratospheric origin and ozone which has been photochemically produced. Within the scope of a case study, the exchange processes in the vicinity of a tropopause fold was investigated from Eulerian and Lagrangian point of view. Air parcels, entering the troposphere due to turbulent and diabatic processes along the tropopause fold, are transported deep into the lower troposphere. Simulated ozone enhancements at ground level, caused by descending stratospheric air masses, are evaluated with observational data. It is shown that the results of the finer resolved, regional model instance coincide well with the measurements. Within the framework of a Lagrangian study, mixing time-scales for STT-processes are determined. It is shown, that air parcels, staying in the troposphere longer than ten hours, influence the troposphere by inserting their stratospheric tracer properties and are therefore not negligible. Furthermore, the effectivity of mixing along a tropopause fold is analysed, showing that almost all of the air masses being once in the tropopause fold are transported into the troposphere during the following two days.
Publications
- Austauschprozesse an Tropopausenfalten extratropischer Zyklonen (Exchange processes at tropopause folds of extratropical cyclones) Dissertation am Fachbereich Physik, Mathematik und Informatik der Johannes Gutenberg-Universität in Mainz
Christiane Hofmann
- Stratosphere-troposphere exchange in the vicinity of a tropopause fold. Atmos. Phys. Chem. Discuss.
Hofmann, C., Kerkweg, A., Hoor,P and Jöckel, K.
(See online at https://doi.org/10.5194/acp-2015-949)
