Mixing at the extratropical tropopause as characterized by collocated airborne H2O and O3 lidar observations
The composition of the extratropical transition layer (ExTL), which is the transition zone between the stratosphere and the troposphere in the mid-latitudes, largely depends on dynamical processes fostering the exchange of air masses. Here we follow the need to better characterize the ExTL in relati...
| Main Authors: | , , |
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| Format: | Text |
| Language: | English |
| Published: |
2020
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| Subjects: | |
| Online Access: | https://doi.org/10.5194/acp-2020-1085 https://acp.copernicus.org/preprints/acp-2020-1085/ |
| Summary: | The composition of the extratropical transition layer (ExTL), which is the transition zone between the stratosphere and the troposphere in the mid-latitudes, largely depends on dynamical processes fostering the exchange of air masses. Here we follow the need to better characterize the ExTL in relation to the dynamic situation using the first-ever collocated airborne lidar observations of ozone (O 3 ) and water vapour (H 2 O) across the tropopause. The potential of such lidar profile data, required for a novel, two-dimensional depiction of the complex trace gas distributions and mixing along cross-sections, is illustrated for a perpendicular jet stream crossing during a research flight over the North Atlantic conducted on 1 October 2017 in the framework of the Wave-driven Isentropic Exchange (WISE) field campaign. The analysis of the ExTL shape and composition uses a combined view of the lidar data in geometrical and tracer-tracer (T-T) space, which was so far not possible from existing observations. For this particular case study, which is considered to be representative for the climatological distribution, the T-T depiction allows to identify distinct mixing regimes that suggest mixing of air masses with differing origin: we find clearly separated mixing of stratospheric air with moist extratropical air as well as with dry tropical air in the surrounding of the extratropical jet stream. This separation is indicative for differing transport pathways in the troposphere which need to be further elaborated using Lagrangian diagnostics. The O 3 and H 2 O distributions confirm strongest mixing above and below the maximum jet stream winds, while it is suppressed in-between. The interrelation of chemical and dynamical discontinuities is investigated and strongest isentropic trace gas gradients are found to be better correlated with maximum isentropic PV gradients than with classical dynamical tropopause definitions. Although the methods neither allow conclusions on the individual mixing process nor on the location and time of the event, the consideration of data subsets allows discussing the formation and interpretation of isentropic and vertical mixing lines in T-T space and to develop hypotheses on mixing at different time-scales. The presented two-dimensional lidar data is considered to be of relevance for the investigation of further synoptic situations leading to mixing across the tropopause and for future validation of chemistry and numerical weather prediction models. |
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