A process based definition of the extratropical transition layer

The existence of the extratropical transition layer (ExTL) still lacks a physical process based explanation. It is yet an empirical finding based on the observed gradient changes of species within the lowermost stratosphere (LMS). The trace gas distribution directly results from localized and transient dynamical processes in the tropopause region. However, up to this point a dynamic- or process based definition of the ExTL remains an open research question, to large parts due to limitations of the available model and measurement data and resulting uncertainties in the assessment of the relevance of individual processes.

 

We present a synopsis of a series of recent research studies, which provide strong indications that tropopause-based wind shear and resulting dynamic instability is the key process for the formation of the ExTL. For this we use a multitude of different model and observational data sets, thus addressing the formation and maintenance of the ExTL from the chemical view, the process based dynamical view as well as from the large scale synoptics:

• Process studies identify dynamic instability and turbulent cross-tropopause mixing based on trace gas measurements within distinct mesoscale layers of exceptional wind shear at the tropopause.

• The analysis of several years of ECMWF ERA5 reanalysis data shows that these vertically confined transient layers of intense shear are a frequently occurring global scale feature, which is forced by the underlying large scale tropospheric dynamics.

• The significance of this wind shear layer as a persistent source for dynamic instability and turbulent breakdown of the flow at the tropopause is validated with several years of operational EDR turbulence measurements from commercial aircraft.