The extratropical tropopause – Trace gas perspective on tropopause definition choice

Abstract. Aircraft measurement campaigns such as IAGOS-CARIBIC and HALO missions are invaluable sources of trace gas observations in the extratropical Upper Troposphere and Lower Stratosphere (exUTLS), providing simultaneous measurements of multiple substances. To contextualise these observations, the use of dynamic coordinate systems such as tropopause-relative coordinates is highly beneficial. Different approaches to define the tropopause are commonly used in studies, based on either differences in chemical composition, dynamical parameters, or temperature gradients between the troposphere and stratosphere. We examine how different tropopause definitions influence the climatology and seasonality of trace gas observations. Meteorological parameters used in this analysis are obtained from ERA5 reanalysis data interpolated to the flight tracks. Our findings indicate that the thermal tropopause results in larger variability near the tropopause. Different potential-vorticity thresholds result in vertically displaced distributions but similar seasonal variability around the tropopause. Of these, the 3.5 PVU threshold best represents the transport barrier at the tropopause as indicated by the sharpest cross-tropopause gradient. Tracer-based tropopauses using O₃ or N₂O can be used effectively to differentiate between the troposphere and stratosphere without the use of additional model data. A chemical tropopause tied to a mid-latitude ozone climatology was shown to return a meaningful tropopause-relative coordinate. An investigation of individual flights showed that the tropopauses calculated from model data did not represent small-scale structures well, while the 'in-situ' chemical tropopauses provided more meaningful results. For the calculations of an N₂O-based statistical tropopause, however, the case studies highlighted the importance of carefully setting initial parameters.