Evaluating the impact of tropopause definitions on long-lived tracer distributions in the exUTLS

Aircraft measurement campaigns such as IAGOS-CARIBIC and HALO missions are invaluable sources of long-lived trace gas observations in the extratropical Upper Troposphere and Lower Stratosphere (exUTLS). The simultaneous measurement of multiple substances enables a comprehensive characterisation of sampled air masses.

To contextualize these observations, the use of dynamic coordinate systems - where measurements are for example presented relative to the tropopause - is highly beneficial. The tropopause itself can be defined from several perspectives, including differences in chemical composition, dynamical parameters, or temperature gradients between the troposphere and stratosphere.

In this study, we examine how different tropopause definitions influence the climatology of long-lived tracer substance observations. We investigate how effective filtering of tropospheric and stratospheric air masses can homogenise measurements of long-lived tracers and therefore decouple atmospheric dynamics from long-term trends and seasonalities. Meteorological parameters used in this analysis are obtained from ERA5 reanalysis data sets, which have been subsampled along the flight tracks.

Our findings indicate that the thermal tropopause results in larger variability in bins around the tropopause. Different potential-vorticity thresholds result in vertically displaced distributions but similar trends around the tropopause. Chemical tropopauses, while effective in differentiating between the troposphere and stratosphere, show significant limitations in their sensitivity towards the surface.