Impact of turbulent mixing on the UTLS chemistry and radiation in chemistry climate models
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Abstract
The upper troposphere lower stratosphere (UTLS) plays an important role on Earth's climate by affecting the Earth's radiation budget. Surface temperature is sensitive to changing radiatively active gases in the UTLS region. Stratosphere-troposphere exchange (STE), the bi-directional exchange process between the stratosphere and troposphere is the major pathway of exchange between both spheres. Turbulent mixing by clear air turbulence (CAT), as a form of STE, could rapidly mix the chemical species between the stratosphere and troposphere, especially near the tropopause and jet stream. CAT is also expected to intensify under climate change. Considering the importance of STE and its link with turbulent mixing, and the increasing trend of CAT, understanding and representing the process on how turbulent mixing could redistribute the UTLS chemistry and its corresponding impact on Earth's radiation budget is becoming a crucial task.
The first study presented in this thesis introduced an enhanced vertical setup EH-84 with high vertical resolution in the UTLS for the multi-scale climate chemistry model MECO(n), and a novel diagnostic delta tracer-tracer correlation. This new vertical setup provides a suitable tool to understand and quantify the bidirectional cross-tropopause transport and allows a more detailed analysis on small-scale processes. The delta tracer-tracer correlation also provides a new method on separating the sole impact of a single process in a model simulation. This study focuses on the impact of turbulent mixing on tracers in the UTLS. The result shows that the enhanced setup is able to capture several distinct turbulent mixing events in the UTLS with different characteristics induced by turbulence and strong vertical tracer gradient.
The second study presented in this work investigates the sensitivity of turbulent mixing by CAT on the UTLS chemistry and its radiative impact. It is done by implementing a new submodel CAT in the climate chemistry model EMAC. This study introduced a new turbulence diagnostic MoCATI based on vertical wind shear, deformation, divergent trend and static stability, as the mixing coefficient of the CAT mixing scheme. Simulation result shows that ozone in the UTLS is significantly reduced by 10 to 20\% after enabling the CAT submodel. It is a result of not only the pure physical mixing but also the chemical feedback of other tracers mixed by CAT. The redistribution of tracers by CAT also changed the chemical regime of ozone and the methane lifetime. The result also shows the turbulent mixing leads to radiative cooling at the top of the atmosphere (TOA) for about 0.2 W/m$^{2}$.
The third study extends the CAT mixing scheme of EMAC from only applied on tracers to applied on water and temperature as well. Result shows a significant increase of water vapour near the tropopause, leads to a significant radiative heating of 0.79 W/m$^{2}$.
The fourth study prepares the simulation setup for the TPEx I measurement campaign of TPChange, examines the synoptic representation of the multi-scale climate chemistry model MECO(n). Result shows the coarse horizontal resolution of EMAC within MECO(n) could lead to deviation of synoptic features in the nested COSMO instances, leading to difficulties in direct comparison with measurements. Increasing the EMAC horizontal resolution could improve the representation of synoptic features.
This thesis enhanced our knowledge on the role of turbulent mixing on tracers in the UTLS by providing tools to investigate the representation of small-scale process e.g. turbulent mixing in MECO(n). By parametrizing turbulent mixing in EMAC, it enhanced the understanding of how clear air turbulence could shape the UTLS chemistry and radiation. Finally, the investigation on the MECO(n) representation enhanced the understanding on how the model dynamics respond to different model resolutions.