Impact of upper tropospheric jet-front sytems on the mesoscale structure of the tropopause inversion layer and cross-tropopause transport
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Abstract
The tropopause represents the transition region between the first two layers of the atmosphere, i.e., the troposphere and the stratosphere. It can be regarded as a quasi- permeable transport barrier that allows exchange and mixing of air only for certain processes. Such non-conservative processes cause material changes in the potential vorticity, a fluid-dynamic conservation property for air parcels that are generally approximated as closed thermodynamic systems. Thus, the transition from characteristic tropospheric to stratospheric potential vorticity and vice vera is equatable with transport of air through the tropopause. The significance of dynamic instability and turbulence as a non-conservative mixing process varies depending on the underlaying synoptic situation. In this context, the role of the tropopause inversion layer is not finally understood. This layer is defined based on a local maximum in static stability, a quantity which to a large degree determines the vertical distribution of potential vorticity in the tropopause region. On one hand, much of what is known about the tropopause inversion layer and its relation to the dynamic stability of the flow is based on numerical model studies where the results depend significantly on the degree of idealization as well as on the representation of the physical processes that induce dynamic instability. On the other hand, there is a lack of high resolution in situ measurements specifically dedicated to the investigation of the relation between troposphere-stratosphere exchange and the tropopause inversion layer, and thus, also the validation of the numerical models. The present work adresses this issue with the central aim to investigate mixing processes at the tropopause, particularly in relation to the tropopause inversion layer. For this a set of analyses on different spatial and temporal scales is performed. The analyses range from individual case studies based on measurements from the airborne research campaign WISE that took place during 2017 over the North Atlantic, to model-based process studies of baroclinic life cycles, up to climatological scales based on ten years of northern hemispheric reanalysis data.
The analyses across all scales reveal that turbulent mixing occurs particularly in regions which are characterised by a pronounced tropopause inversion layer. The underlaying dynamic instability is forced by a layer of strong vertical wind shear which is located closely above the tropopause. On synoptic scales the wind shear layer and the tropopause inversion layer emerge simultaneously in ridges of baroclinic waves. Furthermore, each layer exhibits a distinct mesoscale variability which is linked to differences in the mechanisms that influence the evolution of each layer. The wind shear layer as a tropopause-based phenomenon occurs on global and climatological scales. For latitudes that are not dominated by baroclinic wave dynamics, several regions have been identified that favor the occurrence of strong wind shear near to the tropopause: Over the Asian continent and associated with the subtropical jet stream, over the Indian Ocean and associated with the Asian summer monsoon circulation, and over the maritime continent and associated with the El Niño Southern Oscillation ocean–atmosphere coupling.
The occurrence of the tropopause wind shear layer is associated with low Richardson numbers in the lower stratosphere, and thus, an increased potential for dynamic instability. In the midlatitudes these preconditions for turbulent mixing occur in regions of high tropopause altitudes like ridges of baroclinic waves, i.e., a region that has gained comparatively little attention in the context of research on stratosphere-troposphere exchange. The vertically confined occurrence of the tropopause wind shear layer within the first 1–2 kilometers above the local tropopause indicates a relation to the extratropical transition layer. The extratropical transition layer is defined based on distinct trace gas gradients which are shaped by mixing processes, and its occurrence is limited to the same vertically confined region. Furthermore, the analysis highlights the significance of the mesoscale variability of both static stability and wind shear, despite the global scale and climatological character of the tropopause inversion layer and the wind shear layer.
In summary, the analysis narrows down the significance of shear-induced turbulent mixing in the lower stratosphere. The preconditions for local small-scale turbulent mixing in the tropopause region are frequently met from the tropics up to high latitudes, which is associated with the dominant tropospheric large scale dynamics within each meridional region. This in turn affects the chemical composition of the tropopause region which can have a significant impact on the global radiative budget.