Upward transport into and within the Asian monsoon anticyclone as inferred from StratoClim trace gas observations
<p>Every year during the Asian summer monsoon season from about mid-June to early September, a stable anticyclonic circulation system forms over the Himalayas. This Asian summer monsoon (ASM) anticyclone has been shown to promote transport of air into the stratosphere from the Asian tropospher...
| Main Authors: | , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/1267/2021/acp-21-1267-2021.pdf |
| Summary: | <p>Every year during the Asian summer monsoon season from
about mid-June to early September, a stable anticyclonic circulation system
forms over the Himalayas. This Asian summer monsoon (ASM) anticyclone has
been shown to promote transport of air into the stratosphere from the Asian
troposphere, which contains large amounts of anthropogenic pollutants.
Essential details of Asian monsoon transport, such as the exact timescales
of vertical transport, the role of convection in cross-tropopause exchange,
and the main location and level of export from the confined anticyclone to
the stratosphere are still not fully resolved. Recent airborne observations
from campaigns near the ASM anticyclone edge and centre in 2016 and 2017, respectively, show a steady decrease in carbon monoxide (CO) and increase in
ozone (O<span class="inline-formula"><sub>3</sub></span>) with height starting from tropospheric values of around 100 ppb CO and 30–50 ppb O<span class="inline-formula"><sub>3</sub></span> at about 365 K potential temperature. CO
mixing ratios reach stratospheric background values below <span class="inline-formula">∼25</span> ppb at about 420 K and do not show a significant vertical gradient at higher
levels, while ozone continues to increase throughout the altitude range of
the aircraft measurements. Nitrous oxide (N<span class="inline-formula"><sub>2</sub></span>O) remains at or only
marginally below its 2017 tropospheric mixing ratio of 333 ppb up to about
400 K, which is above the local tropopause. A decline in N<span class="inline-formula"><sub>2</sub></span>O mixing
ratios that indicates a significant contribution of stratospheric air is
only visible above this level. Based on our observations, we draw the
following picture of vertical transport and confinement in the ASM
anticyclone: rapid convective uplift transports air to near 16 km in
altitude, corresponding to potential temperatures up to about 370 K.
Although this main convective outflow layer extends above the level of zero
radiative heating (LZRH), our observations of CO concentration show little
to no evidence of convection actually penetrating the tropopause. Rather,
further ascent occurs more slowly, consistent with isentropic vertical
velocities of 0.7–1.5 K d<span class="inline-formula"><sup>−1</sup></span>. For the key tracers (CO, O<span class="inline-formula"><sub>3</sub></span>, and
N<span class="inline-formula"><sub>2</sub></span>O) in our study, none of which are subject to microphysical
processes, neither the lapse rate tropopause (LRT) around 380 K nor the cold
point tropopause (CPT) around 390 K marks a strong discontinuity in their
profiles. Up to about 20 to 35 K above the LRT, isolation of air inside the
ASM anticyclone prevents significant in-mixing of stratospheric air (throughout this text, the term in-mixing refers specifically to mixing processes that introduce stratospheric air into the predominantly tropospheric inner anticyclone). The
observed changes in CO and O<span class="inline-formula"><sub>3</sub></span> likely result from in situ chemical processing.
Above about 420 K, mixing processes become more significant and the air
inside<span id="page1268"/> the anticyclone is exported vertically and horizontally into the
surrounding stratosphere.</p> |
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| ISSN: | 1680-7316 1680-7324 |