Cirrus related diabatic processes and impact on ExTL structure

The extratropical transition layer (ExTL), located above the extratropical tropopause, is an atmospheric region characterized by vertical gradients in atmospheric composition, transitioning between tropospheric and deeper stratospheric properties. Since the tropopause constitutes a mixing barrier under adiabatic conditions, the existence of the ExTL is evidence of diabatic processes influencing this atmospheric region.

Although not usually thought to occur above the tropopause, cirrus clouds can be the cause of diabatic processes via radiative and latent (microphysical) effects. Here we present two cases of cirrus occurrence above the extratropical tropopause captured during the AIRTOSS-ICE (2013) and TPEx (2024) campaigns. The observational data include simultaneous in situ measurements on two platforms in different altitudes, allowing for the calculation of vertical gradients of potential temperature (static stability) and other quantities. The measurements are supported by ERA5 reanalysis data as well as Lagrangian analyses of ICON (Icosahedral Nonhydrostatic) model simulations which yield contributions from different diabatic processes to the evolution of air masses.

The results of the AIRTOSS-ICE case suggest long residence times of the cirrus in stratospheric air as well as substantial differences in static stability between in- and outside the cirrus. The second case confirms the earlier observations and extends the simultaneous in situ measurements to ozone mixing ratios. Our findings underline the importance of diabatic cloud processes for the thermodynamic structure of the ExTL and potential cross tropopause exchange.