The UTLS region can be regarded as a transition of transport regime and time scales. The troposphere is charecterized by diabatic and turbulent transport processes occurring on short time scales, whereas the stratosphere is dominated by the large scale Brewer-Dobson circulation (BDC) with time scales ranging up to years. Importantly the BDC can be regarded as the slow wave driven residual circulation and rapid quasi isentropic eddy mixing. Both act on the distribution of tropospheric source gases. These enter the stratosphere predominately in the tropical tropopause layer (TTL). The extratropical part of the UTLS is in addition affected by a number of mixing processes on different scales. These comprise isentropic mixing at the jets, convection and gravity wave breaking.
The tropopause itself is not a material conserved surface, instead it is highly variable since it is affected by diabatic processes, from the turbulent and cloud scale to the planetary scale. The variability of the tropopause and range of scales of mixing and transport processes impose strong difficulties in temporal variability on the seasonal scale or long-term trends.
This also holds for the identification and quantification of exchange processes.
Therefore, we use combinations of tracer measurements of constituents with relatively well known sources and sinks like (CO and N2O), to construct tropopause based climatologies or perform tropopause based process analysis to reduce the effect of tropopause variability. We perform measurements of CO, N2O, CH4, and CO2 onboard the new german aircraft HALO (High Altitude and Long Range Research Aircraft): TACTS/ESMVAL, POLSTRACC, WISE, SOUTHTRAC, the GFD LearJet (SPURT-project, AIRTOSS) or the DLR Falcon (GW Lifecycle, DEEPWAVE).
The measurements are complemented by Lagrangian and Eulerian analysis including artificial tracers of different type, which are sensitive to e.g. source regions, transport times, etc. Comparing the in-situ observations with such tracers allows to quantify or constrain transport times, air mass fraction and the identification of specific transport processes or regimes. Climatologies of aircraft data with satellite observations provide the opportunity to extend the sparse, but precise airborne observations with a global coverage observation to quantify the long-term evolution of atmospheric constituents (OCTAV-UTLS).