Tracking air mass history with respect to convective mixing - a SOUTHTRAC example

The composition of the upper troposphere is determined by both the tropospheric transport times causing near surface air to be transported away from the emission sources as well as the dynamics of the atmosphere, including exchange with the stratosphere. Atmospheric convection and the associated tracer transport lead to rapid mixing of air masses given the strong up- and downdrafts in convective clouds, both in the tropics and the midlatitudes.

However, determining the impact of convection on a sampled air mass is rather difficult from an experimental point of view, as the mixing ratios of short lived compounds can originate from one individual or several convective mixing events. For that purpose, we use global simulation results using parameterised convection and driven by re-analysis data for the period of the SOUTHTRAC campaign.
We sample the output from the convective exchange matrix, i.e., a novel diagnostic for convective transport, on backward trajectories for aircraft measurements and therefore can identify the contribution of near surface air to the UTLS composition. Additionally, we analyse the time evolution of the air mass and the convective events to estimate the contribution of observed trace gases to the total composition in an temporally integrated point of view, which allows to also estimate the chemical degradation of compounds given their individual chemical lifetimes. Overall, this study which is within the framework of the TPChange project, consequently leads to a better understanding of the composition of the upper troposphere and potential injections into the lower stratosphere.