Multiscale Dynamical Processes Shaping a Mixing Line

The paper discusses multiscale dynamical processes shaping a mixing line in the upper troposphere/lower stratosphere (UTLS). It focuses on aircraft observations above southern Scandinavia during a mountain wave event and how they can be analyzed based on dynamic variables and the trace gases $N_2$O and CO. This study aims to identify the irreversible component of the stratosphere-troposphere exchange. It was shown that the overall shape of the mixing line is determined by the large-scale and mesoscale atmospheric conditions in the UTLS. Especially, the wide range of $\Theta$ values along the flight tracks causes a compact, almost linear tracer-tracer relation between $N_2$O and CO. Only motion components with scales less than 4 km lead to the observed scatter around the mixing line. The anisotropic and patchy nature of the observed turbulence is responsible for this scatter in $N_2$O and CO. The turbulence analysis reveals different scaling laws for the power spectra upstream, over the ridge and downstream of the mountains that lead to energy dissipation and irreversible mixing. The study suggests that turbulence dynamics may follow a cycle starting with 3D homogeneous isotropic turbulence upstream, transitioning to anisotropic turbulence over the ridge and further downstream. This transition is attributed to an interplay between turbulent eddies and internal gravity waves.