Fine scale structure of the tropopause region as measured during the TPEx mission

The extratropical transition layer of (ExTL) has been recognized about 20 years ago as part of the upper troposphere / lower stratosphere (UTLS) of the extratropics, which shows the chemical characteristics of both, the stratosphere and the troposphere. Its composition is to a large extent dominated by rapid transient small scale dynamical processes, driven by mid-latitude synoptics. The dynamical processes may include frontal uplift and shear at the tropopause , convection and gravity wave breaking, all in sum leading to turbulence and mixing. As a result gradients of tracer, moisture, and aerosol are highly perturbed on small scales and cross tropopause exchange may occur.

Here we report on a novel measurement approach during June 2024 with a double platform airborne approach targeting small scale variability in the UTLS. This allowed for simultaneous measurements of ozone, humidity and aerosol size distribution on two platforms, which provide information on the vertical gradients of these quantities. The gradients of ozone and aerosol number concentration show a surprisingly high variability in the UTLS and at the tropopause highlighting the importance of small scale processes for the composition of the tropopause region. The measurements were complemented by surface-based and balloon-borne observations at one surface site and comprehensive forecasts including forward trajectories based on ICON forecast data as well as ECMWF forecast data.

We will present highlights from the mission including convective overshoots deep into the LMS at high latitudes, probing of convective outflow and mixing at the tropopause, as well as cirrus occurrence in the stratosphere.

The mission was a central part of the collaborative research center TPChange (The tropopause region in a changing atmosphere).