Turbulent mixing at the polar jet and transport of cirrus particles into the lowermost extratropical stratosphere

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Abstract. The composition of the extratropical transition layer (ExTL), in particular the mixing ratios of ozone and water vapour, has a high impact on the radiative budget of the atmosphere. The ExTL is a transition region above the extratropical tropopause, characterized by smooth vertical gradients between typical tropospheric and stratospheric properties. It is shaped by several transport pathways, including quasi-isentropic mixing at the subtropical jet, diabatic descent from the stratosphere as part of the overturning circulation, and diabatic cross-tropopause exchange. The latter pathway is partially suppressed by the strong static stability above the tropopause, so that diabatic processes are necessary 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 transport of ice particles through turbulent mixing across the tropopause caused by strong wind shears above the jet stream. The measurements are complemented by Lagrangian analyses of ICON model simulations for atmospheric context and history. High resolution acceleration measurements indicate turbulence while Lagrangian analysis shows suitable conditions for the turbulent mixing one hour prior to the measurements. Trace gas correlations and ice particle measurements confirm cross tropopause mixing through simultaneous occurrence of a stratospheric chemical signature and ice particles in subsaturation. With this combined analysis of measurements and simulations, we demonstrate that turbulent mixing is a potential pathway across the extratropical tropopause and that ice particles mixed into the stratosphere by this process possibly constitute a source of stratospheric water.

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