Abstract
Abstract. Observations of ozonesonde measurements of the NDACC/SHADOZ (Network for the Detection of Atmospheric Composition Change and the Southern Hemisphere ADditional OZonesondes) program and humidity profiles from the daily Météo-France radiosondes at Réunion island (21.1∘ S, 55.5∘ E) from November 2013 to April 2016 were analyzed to identify the origin of wet upper-tropospheric air masses with low ozone mixing ratio observed above the island, located in the southwest 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 back trajectories 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 it is observed more than 1000 km away from the island, in the tropics, either from tropical storms or the Intertropical Convection Zone (ITCZ). In November and December, the air masses above Réunion island originate, on average, from central Africa and the Mozambique Channel. During January and February the source region is the northeast of Mozambique and Madagascar. Those results improve our understanding of the impact of the ITCZ and tropical cyclones on the hydration of the upper troposphere in the subtropics in the SWIO.
Highlights
The variability of ozone in the tropical upper troposphere (10–16 km in altitude) is important for the climate as it influences the radiative budget (Lacis et al, 1990; Thuburn and Craig, 2002; Riese et al, 2012) and modifies the oxidizing capacity of the atmosphere and the lifetime of other chemical species
We analyze ozonesonde measurements of the NDACC/Southern Hemisphere ADditional OZonesondes (SHADOZ) program and humidity profiles from daily Météo-France radiosondes from Réunion island between November 2013 and April 2016 to identify the origin of wet upper tropospheric air masses with low ozone mixing ratio observed above the island, and we try to understand the role of transport, detrainment and mixing processes on the composition of the tropical upper troposphere over Réunion island
The value is in agreement with a climatological water vapor mixing ratio (WV) value computed with the Microwave Limb Sounder (MLS) v4.2 water vapor data for 2005–2017
Summary
The variability of ozone in the tropical upper troposphere (10–16 km in altitude) is important for the climate as it influences the radiative budget (Lacis et al, 1990; Thuburn and Craig, 2002; Riese et al, 2012) and modifies the oxidizing capacity of the atmosphere and the lifetime of other chemical species. Solomon et al (2005) used a statistical method to characterize the impact of convection on the local ozone minimum in the upper troposphere above the SHADOZ sites within the maritime continent (Fiji, Samoa, Tahiti and Java) They identified a minimum of 20 ppbv of ozone in 40 % of the ozone profiles. We analyze ozonesonde measurements of the NDACC/SHADOZ program and humidity profiles from daily Météo-France radiosondes from Réunion island between November 2013 and April 2016 to identify the origin of wet upper tropospheric air masses with low ozone mixing ratio observed above the island, and we try to understand the role of transport, detrainment and mixing processes on the composition of the tropical upper troposphere over Réunion island.
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