Daniel Kunkel

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.

Determining the atmospheric abundance and the emission sources of carbonyl sulfide (OCS) plays a crucial role for the sulfur budget in the atmosphere. In addition to its main anthropogenic and biogenic sources, biomass burning is assumed to be an essential, but not well constrained source of OCS. From November 2019 onward extensive bushfires along the eastern coast of Australia released huge amounts of trace species into the atmosphere; during one flight of the SouthTRAC campaign air masses were observed carrying signatures of these fires along the southern coast of South America. An analysis of the composition of air masses inside and outside the plume revealed distinct differences in the trace species abundances. Specifically, our results indicate that OCS shows no enhancement in its atmospheric mixing ratio due to the Australian fires and thus highlights the varying emission strength of OCS for different types of biomass burning.

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.

This study focuses on the spatial and temporal co-occurrence of gravity waves (GWs) and of vertical shear and turbulence in the extratropical lowermost stratosphere (LMS). For this, one year of ERA5 reanalysis data is used to analyse the occurrence and variability of resolved GW momentum fluxes, vertical shear, and turbulence indices in the region of the North Atlantic. In the tropopause region GWs can lead to strong vertical wind shear and ultimately to the generation of turbulence, which may result in troposphere-stratosphere exchange and mixing and is a risk for commercial aviation. The occurrences of GWs, shear, and turbulence are analysed in terms of vertical, geographical, and seasonal occurrence frequency distribution and put in relation to the large-scale circulation and to processes related to GW appearances. The contribution of resolved GWs to shear is found to be notable, especially in the North Atlantic winter lowermost stratosphere, where vertical momentum flux convergence provides a peak zonal GW forcing up to −2 m s^-1 day^-1 around 45° N at tropopause altitudes. The prominent vertical propagation in the wintertime mid-latitudes substantially leads to the formation of belt-like structures of GW activity, as evident by momentum fluxes, and further contributes to the pronounced occurrence of shear in the LMS. Ultimately, this study discusses the role of small-scale dynamics in shaping a quasi-permanent layer of elevated shear above the extratropical tropopause and its potential to generate turbulence in this region.

This study focuses on the spatial and temporal co-occurrence of gravity waves (GWs) and of vertical shear and turbulence in the extratropical lowermost stratosphere (LMS). For this, one year of ERA5 reanalysis data is used to analyse the occurrence and variability of resolved GW momentum fluxes, vertical shear, and turbulence indices in the region of the North Atlantic. In the tropopause region GWs can lead to strong vertical wind shear and ultimately to the generation of turbulence, which may result in troposphere-stratosphere exchange and mixing and is a risk for commercial aviation. The occurrences of GWs, shear, and turbulence are analysed in terms of vertical, geographical, and seasonal occurrence frequency distribution and put in relation to the large-scale circulation and to processes related to GW appearances. The contribution of resolved GWs to shear is found to be notable, especially in the North Atlantic winter lowermost stratosphere, where vertical momentum flux convergence provides a peak zonal GW forcing up to −2 m s^-1 day^-1 around 45° N at tropopause altitudes. The prominent vertical propagation in the wintertime mid-latitudes substantially leads to the formation of belt-like structures of GW activity, as evident by momentum fluxes, and further contributes to the pronounced occurrence of shear in the LMS. Ultimately, this study discusses the role of small-scale dynamics in shaping a quasi-permanent layer of elevated shear above the extratropical tropopause and its potential to generate turbulence in this region.

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.

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.

Small-scale dynamics, particularly gravity waves (GWs), modify vertical wind shear and can trigger turbulence in the lowermost stratosphere (LMS), thereby influencing the transport of trace species across the tropopause. Although idealized modeling and observational case studies have demonstrated this link, the contribution of small-scale dynamics to turbulence generation remains poorly understood, particularly under real atmospheric conditions. Here, we investigate the relationship between GWs, vertical wind shear, and clear-air turbulence (CAT) in the LMS over the North Atlantic during a baroclinic life cycle. We combine airborne observations with ERA5 reanalysis and high-resolution forecasts from the IFS and ICON models. To isolate the contribution of small-scale dynamics to turbulence generation, we extract the small-scale divergent component of the modeled wind field. From this, we derive momentum flux, perturbation vertical wind shear, and the turbulence indices TI1 and TI2, and compare these quantities with those calculated from the full wind fields. Trace-species mixing is observed within a region of enhanced GW activity over Iceland that is characterized by increased vertical wind shear and turbulence. ERA5 reproduces the spatial and temporal distribution of enhanced shear and turbulence but underestimates shear magnitudes relative to the forecasts while yielding comparable values of TI1 and TI2. These findings extend previous idealized studies from Umbarkar and Kunkel (2025) to real atmospheric conditions and provide further evidence that GW-induced small-scale dynamics contribute to turbulence generation in the LMS. They also demonstrate the potential of ERA5 for long-term investigations of the role of small-scale dynamics in transport and mixing near the tropopause.

During the SOUTHTRAC mission in autumn 2019 elevated mixing ratios of carbon monoxide (CO), carbon dioxide CO ₂ , nitrogen oxide (NO) and total reactive nitrogen NO _y were observed during a flight at the beginning of October. The potential plume extended over more than 1000 km (15° latitude) east of the Brasilian coast at altitudes of 13 km in the upper troposphere. In-situ measurements showed elevated ozone in this plume (≈ 100 ppbv), being 20–40 ppbv higher than during a previous flight in early September at exactly the same flight route. For the plume flight positive correlations of ozone and pollutants (CO, NO, NO _y ) indicate ozone production in these pollution layers. Lagrangian Analysis shows, that the observed air masses were strongly affected by biomass burning over Amazonia. A combined analysis of a chemical Lagrangian box model and a global chemistry climate model (EMAC) revealed that ozone production from biomass burning predominantly caused the ozone enhancements. The effect is intensified by NO _x produced from lightning. Upward transport of the plumes happened ≈ one week before the flight, allowing ozone to be formed and enhanced by 25 % compared to the September flight. Estimate of the potential climate impact show, that the biomass burning produced ozone has an impact on the radiation budget, namely a spatially and regionally localized radiative flux disturbance of up to 250 mW m ⁻² at the tropopause and 150 mW m ⁻² at the top of the atmosphere.

This study focuses on the spatial and temporal co-occurrence of gravity waves (GWs) and of vertical shear and turbulence in the extratropical lowermost stratosphere (LMS). For this, one year of ERA5 reanalysis data is used to analyse the occurrence and variability of resolved GW momentum fluxes, vertical shear, and turbulence indices in the region of the North Atlantic. In the tropopause region GWs can lead to strong vertical wind shear and ultimately to the generation of turbulence, which may result in troposphere-stratosphere exchange and mixing and is a risk for commercial aviation. The occurrences of GWs, shear, and turbulence are analysed in terms of vertical, geographical, and seasonal occurrence frequency distribution and put in relation to the large-scale circulation and to processes related to GW appearances. The contribution of resolved GWs to shear is found to be notable, especially in the North Atlantic winter lowermost stratosphere, where vertical momentum flux convergence provides a peak zonal GW forcing up to −2 m s^-1 day^-1 around 45° N at tropopause altitudes. The prominent vertical propagation in the wintertime mid-latitudes substantially leads to the formation of belt-like structures of GW activity, as evident by momentum fluxes, and further contributes to the pronounced occurrence of shear in the LMS. Ultimately, this study discusses the role of small-scale dynamics in shaping a quasi-permanent layer of elevated shear above the extratropical tropopause and its potential to generate turbulence in this region.