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Preprints
https://doi.org/10.5194/acp-2020-2
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-2
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Feb 2020

Submitted as: research article | 18 Feb 2020

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A revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Impact of convection on the upper-tropospheric composition (water vapor/ozone) over a subtropical site (Réunion Island, 21.1° S–55.5° E) in the Indian Ocean

Damien Héron1, Stephanie Evan1, Jerome Brioude1, Karen Rosenlof2, Françoise Posny1, Jean-Marc Metzger3, and Jean-Pierre Cammas1,3 Damien Héron et al.
  • 1LACy, Laboratoire de l’Atmosphère et des Cyclones, UMR8105 (CNRS, Université de La Réunion, Météo-France), Saint-Denis de la Réunion, France
  • 2Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, CO, USA
  • 3Observatoire des Sciences de l’Univers de La Réunion, UMS3365 (CNRS, Université de La Réunion, Météo-France), Saint-Denis de la Réunion, France

Abstract. Observations of ozonesonde measurements of the NDACC/SHADOZ program and humidity profiles from the daily Météeo-France radiosondes at Réunion Island (21.1° S, 55.5° E) from November 2013 to April 2016 are analyzed to identify the origin of wet upper tropospheric air masses with low ozone mixing ratio observed above the island, located in the South West Indian Ocean (SWIO). A seasonal variability in hydration events in the upper troposphere was found and linked to the convective activity within the SWIO basin. In the upper troposphere, ozone mixing ratios were lower (mean of 57 ppbv) in humid air masses (RH > 50 %) compared to the background mean ozone mixing ratio (73.8 ppbv). A convective signature was identified in the ozone profile dataset by studying the probability of occurrence of different ozone thresholds. It was found that ozone mixing ratios lower than 45 to 50 ppbv had a local maximum of occurrence between 10 and 13 km in altitude, indicative of the mean level of convective outflow. Combining FLEXPART Lagrangian backtrajectories with METEOSAT 7 infrared brightness temperature products, we established the origin of convective influence on the upper troposphere above Réunion island. It has been found that the upper troposphere above Réunion island is impacted by convective outflows in austral summer. Most of the time, deep convection is not observed in the direct vicinity of the island, but more than a thousand of kilometers away from the island, in the tropics, either from tropical storms or the Inter Tropical Convective Zone. In November and December, the air masses above Réunion Island originate, on average, from Central Africa and the Mozambique channel. During January, February the source region is the North-east of Mozambique and Madagascar. Those results improve our understanding of the impact of the ITCZ and tropical cyclones hydration of the upper troposphere in the subtropics in the SWIO.

Damien Héron et al.

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Interactive discussion

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Status: closed
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Damien Héron et al.

Damien Héron et al.

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Latest update: 05 Jul 2020
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Short summary
Using a statistical method, summers variations (between 2013 and 2016) of ozone and water vapor are characterized in the upper troposphere above Réunion Island (21° S, 55° E). It suggests a convective influence between 9 and 13 km. As deep convection is rarely observed near Réunion Island, this study provides new insights on the long-range impact of deep convective outflow from the intertropical convergence zone (ITCZ) on the upper troposphere over a subtropical site.
Using a statistical method, summers variations (between 2013 and 2016) of ozone and water vapor...
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