Project B04:
Structure formation and mixing in the extratropical tropopause region

Brief Summary

The occurrence of meso- and smaller-scale structures in the midlatitude tropopause region is likely to play an important role for mixing across the tropopause and, thus, for the composition of the lowermost stratosphere. Yet, these structures and processes are not well represented in current weather forecast and climate models. This project focuses on the advective formation of meso- and smaller-scale structures as well as on physical processes that operate on these scales. The key questions to be addressed are: (i) how do the structures form through conservative advection, in particular depending on the resolution of the underlying data as well as on the distance from the tropopause, (ii) how do the physical processes such as cloud physics, radiation, and turbulence affect the structure formation and stratosphere-troposphere exchange, and (iii) how well can coarse-gridded data reproduce tracer structures from high resolution measurements. The project focuses on quasi-horizontal dynamics in the framework of baroclinic life cycles both from idealized simulations with the ICON model and from ERA5 reanalysis data. Lagrangian methods will be applied to study the formation of structures in the advected tracers as well as in potential vorticity. The exchange of mass across the tropopause as well as the formation of the mixing layer in the lowermost stratosphere are investigated using trajectories in combination with a coarse-graining technique as well as with mixing diagnostics based on tracer-tracer correlations. The work will shed new light on the role of small-scale structures for stratosphere-troposphere exchange and for the composition of the lower stratosphere. It will bridge the gap between highly resolved airborne measurements and much coarser resolution numerical models. The results are also expected to provide guidance for future airborne measurements that focus on the extratropical tropopause region.

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