A process based definition of the extratropical transition layer

<p align="left">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.</p> <p>&#160;</p> <p><span lang="en-US">W</span><span lang="en-US">e present a synopsis of a series of </span><span lang="en-US">recent</span> <span lang="en-US">research studies</span><span lang="en-US">,</span> <span lang="en-US">which</span> <span lang="en-US">provide strong indications that tropopause-based wind shear and resulting dynamic instability is </span><span lang="en-US">the</span><span lang="en-US"> key process for the formation of the ExTL. </span><span lang="en-US">For this we use</span><span lang="en-US"> a multitude of </span><span lang="en-US">different </span><span lang="en-US">model and observational</span> <span lang="en-US">data sets, thus </span><span lang="en-US">addressing the formation and maintenance of the ExTL</span><span lang="en-US"> from the chemical </span><span lang="en-US">view</span><span lang="en-US">, the process based </span><span lang="en-US">dynamical </span><span lang="en-US">view as well as </span><span lang="en-US">from </span><span lang="en-US">the large scale synoptics:</span></p> <ul> <li> <p><span lang="en-US">Process studies identify dynamic instability and turbulent </span><span lang="en-US">cross-tropopause </span><span lang="en-US">mixing </span><span lang="en-US">based on trace gas measurements </span><span lang="en-US">within distinct mesoscale layers of exceptional wind shear at the tro</span><span lang="en-US">popause.</span></p> </li> <li> <p><span lang="en-US">The analysis of several years of ECMWF ERA5 reanalysis data </span><span lang="en-US">shows that</span><span lang="en-US"> these </span><span lang="en-US">vertically confined</span><span lang="en-US"> transient</span> <span lang="en-US">layers of intense shear </span><span lang="en-US">are</span><span lang="en-US"> a frequently occurring global scale feature, which is forced by the underlying large scale tropospheric dynamics.</span></p> </li> <li> <p><span lang="en-US">The significance of this wind shear layer </span><span lang="en-US">as a persistent source for dynamic instability </span><span lang="en-US">and turbulent breakdown of the flow at the tropopause</span> <span lang="en-US">is validated with </span><span lang="en-US">several years of </span><span lang="en-US">operational EDR turbulence measurements </span><span lang="en-US">from commercial aircraft.</span></p> </li> </ul>