In situ observations of turbulent cross-tropopause mixing of cirrus particles at the jet stream over the North Sea

The composition of the extratropical transition layer (ExTL), in particular the mixing ratios of ozone and water vapor, has a high impact on the radiative budget of the atmosphere. It is characterized by states originating from merging and mixing of tropospheric and stratospheric characteristics. The transport pathways into the ExTL governing this composition are (1) quasi-isentropic mixing at the subtropical jet with sources in the higher tropical troposphere, (2) diabatic downwelling as part of the overturning circulation with sources in the stratosphere and (3) diabatic transport and mixing across the extratropical tropopause with sources in the extratropical troposphere. The pathway (3) is partially suppressed by the high static stability above the tropopause, such that only strong diabatic processes are able to facilitate mixing between the troposphere and the ExTL.
Here we present in situ measurements taken during the TPEx campaign in summer 2024 over the North Sea, that provide evidence for turbulent mixing across the tropopause caused by strong wind shears above the jet stream. The measurements are complemented by Lagrangian analyses of ICON (icosahedral non‐hydrostatic) model simulations which yield atmospheric context and history of the probed airmasses. We use measurements of N₂O, CO, O₃ and H₂O as well as ice particles to confirm cross-tropopause mixing with special emphasis on the simultaneous occurrence of ice particles in subsaturation and stratospheric chemical signature of the probed air mass. For the identification of turbulence as the responsible process we use high resolution acceleration measurements that are in good agreement with the occurrence and strength of simulated turbulence from ICON. The Lagrangian analysis shows suitable conditions for the turbulent mixing of cirrus particles an hour before the measurements.
This analysis confirms the occurrence of cross-tropopause mixing caused by shear induced turbulence at the jet stream. This process is sufficiently fast to transport ice particles into the strongly subsaturated lower stratosphere where they are sampled before complete evaporation. Thus this turbulent mixing represents a possible transport pathway of tropospheric air, and therefore a source of water, into the ExTL.