Mixing at the extratropical tropopause as characterized by collocated airborne H<sub>2</sub>O and O<sub>3</sub> lidar observations
<p>The composition of the extratropical transition layer (ExTL), which is the transition zone between the stratosphere and the troposphere in the midlatitudes, largely depends on dynamical processes fostering the exchange of air masses. The Wave-driven ISentropic Exchange (WISE) field campaign...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/5217/2021/acp-21-5217-2021.pdf |
| Summary: | <p>The composition of the extratropical transition layer (ExTL), which is the transition zone between
the stratosphere and the troposphere in the midlatitudes, largely depends on dynamical processes
fostering the exchange of air masses. The Wave-driven ISentropic Exchange (WISE) field campaign in
2017 aimed for a better characterization of the ExTL in relation to the dynamic situation. This
study investigates the potential of the first-ever collocated airborne lidar observations of ozone
(<span class="inline-formula">O<sub>3</sub></span>) and water vapor (<span class="inline-formula">H<sub>2</sub>O</span>) across the tropopause to depict the complex trace
gas distributions and mixing in the ExTL. A case study of a perpendicular jet stream crossing with
a coinciding strongly sloping tropopause is presented that was observed during a research flight
over the North Atlantic on 1 October 2017.</p>
<p>The collocated and range-resolved lidar data that are applied to established tracer–tracer (T–T) space
diagnostics prove to be suitable to identify the ExTL and to reveal distinct mixing regimes
that enabled a subdivision of mixed and tropospheric air. A back projection of this information to
geometrical space shows remarkably coherent structures of these air mass classes along the
cross section. This represents the first almost complete observation-based two-dimensional (2D)
illustration of the shape and composition of the ExTL and a confirmation of established conceptual
models. The trace gas distributions that represent typical <span class="inline-formula">H<sub>2</sub>O</span> and <span class="inline-formula">O<sub>3</sub></span> values
for the season reveal tropospheric transport pathways from the tropics and extratropics that have
influenced the ExTL. Although the combined view of T–T and geometrical space does not inform about
the process, location and time of the mixing event, it gives insight into the formation and
interpretation of mixing lines. A mixing factor diagnostic and a consideration of data subsets
show that recent quasi-instantaneous isentropic mixing processes impacted the ExTL above and below
the jet stream which is a confirmation of the well-established concept of turbulence-induced
mixing in strong wind shear regions. At the level of maximum winds reduced mixing is reflected in
jumps in T–T space that occurred over small horizontal distances along the cross section. For a
better understanding of the dynamical and chemical discontinuities at the tropopause, the lidar
data are illustrated in isentropic coordinates. The strongest gradients of <span class="inline-formula">H<sub>2</sub>O</span> and
<span class="inline-formula">O<sub>3</sub></span> are found to be better represented by a potential vorticity-gradient-based tropopause compared to
traditional dynamical tropopause definitions using constant potential vorticity values. The presented 2D lidar data
are considered to be of relevance for the investigation of further meteorological situations
leading to mixing across the tropopause and for future validation of chemistry and numerical
weather prediction models.</p> |
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| ISSN: | 1680-7316 1680-7324 |