Impact of small-scale dynamics on upper troposphere lower stratosphere transport and mixing : contribution of gravity waves to vertical shear
Loading...
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
Reuse License
Abstract
In the extratropical upper troposphere and lower stratosphere (UTLS), the composition is shaped by quasi-horizontal transport often related to the strong upper tropospheric wind, as well as to vertical transport associated with warm conveyor belts and convective systems, and turbulent mixing. These processes are governed by planetary to small-scale dynamics. In the extratropics, mixing processes are tightly linked to dynamics of baroclinic life cycles and more particularly to tropopause folds, cut-off lows and stratospheric streamers. While the large scale dynamics in this context are rather well understood, this can not be said about the associated small scale dynamics. Especially, the role of the latter to the formation of vertical shear of horizontal wind as a potential prerequisite for the occurrence of turbulence in the extratropical tropopause region is not finally understood. One of the processes driving small-scale dynamics are atmospheric gravity waves (GWs), which often occur in relation to baroclinic waves in the extratropics and can potentially dissipate in the UTLS region.
GWs can alter the thermodynamical and dynamical structure of the UTLS by enhancing static stability, thereby increasing shear and the likelihood of dynamic instability and turbulence. These interactions further globally enhance shear in the extratropical tropopause region and result in the formation of the so-called "tropopause wind shear layer". Current understanding of GWs, vertical shear and their role in dynamic instability relies largely on the numerical model studies whose results depend on the degree of idealization and on the representation of the physical processes. In contrast, airborne in situ measurements focusing on the investigation of the relation between small-scale GWs and mixing processes across the tropopause are limited. The present work addresses this issue by investigating the contribution of GWs to transport and mixing processes in the lowermost stratosphere, with a particular focus on the tropopause shear layer.
The impact of GWs on shear is addressed first, by investigating a comprehensive set of idealized baroclinic life cycle experiments using the NWP model ICON. Dry adiabatic simulations across varying spatial resolutions reveal that shear and turbulence occur particularly in regions characterized by pronounced GW activity. Further process understanding is gained from experiments incorporating physical processes like latent heating, (vertical) turbulence, and cloud microphysics. This reveal that tropospheric moist processes significantly influence the extent and occurrence of GWs, shear and potential for turbulence occurrence, mainly driven by latent heat release and stronger baroclinic wave evolution with vigorous vertical motions.
In a subsequent step, the findings from the idealized world are evaluated under realistic atmospheric conditions. The characteristics of GWs and their impact on the distribution of trace species in the lowermost stratosphere are examined for an extratropical cyclone over the North Atlantic using airborne in-situ observations, recent reanalyses and model-forecast datasets. The observed significant correlation between GW momentum flux and enhanced shear perturbations confirms the role of GWs in driving potential turbulence and facilitating trace gas exchange in the lower stratosphere. Further analysis of turbulence diagnostics suggest that GW-induced shear can be regarded as a key mechanism for the occurrence of (clear-air) turbulence in the extratropical lowermost stratosphere.
Finally, the results are put into context of quasi-climatological analysis by means of spatial and temporal co-occurrence of GWs and shear and to the formation of tropopause shear layer in the North Atlantic lowermost stratosphere. It is revealed that the underlying dynamic instability is forced by a layer of strong vertical shear above the tropopause and is strongly influenced by GW activity. The contribution of resolved GWs to shear is found to be notable, especially in winter, where zonal GW forcing peaks at tropopause altitudes. The analysis of these processes highlights the vital role of small-scale dynamics in shaping a quasi-permanent layer of elevated shear above the extratropical tropopause and the occurrence of turbulence in this region.
In summary, this work revisits and refines the hypothesis of Kaluza et al. 2021 by providing more insights into the role of small-scale dynamics, with a focus on GWs, in the formation of the shear layer above the extratropical tropopause. Furthermore, it has been shown that small-scale processes affect the occurrence of turbulence in this region and as such have implications for the dynamics and composition of the tropopause region.