Impact of upper tropospheric jet-front systems on the mesoscale structure of the tropopause inversion layer and cross-tropopause transport [MESO-TIL]
Final Report Abstract
The tropopause is a unique region of the atmosphere, separating the troposphere from the stratosphere. It can be regarded as a semi-permeable layer through which air is only transported under certain conditions. This is of particular interest, since this transport pathway can determine the abundance of harmful trace species in the troposphere and stratosphere. In this project a potential relation between the layer of enhanced static stability, the so-called tropopause inversion layer, and exchange across the tropopause has been studied using data from a numerical weather forecast model along with airborne observations obtained during a field campaign over the North Atlantic. The major goals were to identify i) the variability in the structure of the tropopause over the North Atlantic in regions of baroclinic waves, i.e., the waves which determine our daily weather; ii) the processes which contribute to the tropopause variability as well as to associated cross-tropopause exchange; and iii) the particular role of gravity waves in the formation of the inversion layer as well as on cross-tropopause exchange. The major goals of this project can be summarized as follows: 1. Model-driven analysis of the tropopause inversion layer and the tropopause structure requires a high vertical resolution of underlying model data. Also processes in the troposphere in areas further away from the tropopause need to be represented correctly to fully capture the variability of the tropopause and strength of the inversion layer. 2. A cyclone composite study revealed that predictions from idealized models on the formation of the inversion layer above the tropopause are a generic feature of baroclinic life cycles. Within such life cycles the static stability shows a particular strong enhancement in subtropical air masses which move poleward and are cyclonically wrapped around the cyclone center. 3. The composite study further revealed that the region with enhanced static stability overlaps at least partially with a region of enhanced vertical shear of the horizontal wind. This in turn means that the region of enhanced static stability is prone to become dynamically unstable. Such an instability can lead to turbulence and ultimately mixing across the tropopause and in the lower stratosphere. 4. A detailed analysis of airborne measurements of trace gases along with remote sensing observations of temperature and ECMWF forecast and reanalysis products provided the first direct observation of mixing of air masses in regions of enhanced static stability in the lowermost stratosphere in ridges of baroclinic waves. An analysis of idealized model simulations of baroclinic waves showed that this mixing seems to be a generic feature of baroclinic waves and thus occurs frequently in the extratropical lowermost stratosphere. The depth of the instability has been estimated to be about 600 m. 5. A ten year climatological analysis of the layer of enhanced shear around the tropopause revealed that this shear layer is a very common feature. Maximum shear values emerge in tropical and extratropical regions within the first kilometer above the local thermal tropopause. In the extratropics enhanced shear predominantly emerges in ridges of baroclinic waves, while in the tropics the shear is related to the tropical easterly jet. Overall, the project provided new insights in processes at the extratropical tropopause with a dedicated focus on the tropopause structure and on cross-tropopause transport on small scales, in particular in relation to the so-called tropopause inversion and shear layers.
Publications
- Composite analysis of the tropopause inversion layer in extratropical baroclinic waves, Atmos. Chem. Phys., 19, 6621–6636
Kaluza, T., Kunkel, D., and Hoor, P.
(See online at https://doi.org/10.5194/acp-19-6621-2019) - Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves, Atmos. Chem. Phys., 19, 12607–12630
Kunkel, D., Hoor, P., Kaluza, T., Ungermann, J., Kluschat, B., Giez, A., Lachnitt, H.-C., Kaufmann, M., and Riese, M.
(See online at https://doi.org/10.5194/acp-19-12607-2019)
