Convective hydration in the tropical tropopause layer during the StratoClim aircraft campaign: Pathway of an observed hydration patch

Abstract : The source and pathway of the hydration patch in the TTL (Tropical Tropopause Layer) that was measured during the StratoClim field campaign during the Asian summer monsoon in 2017, and its connection to convective overshoots are investigated. During flight #7, two remarkable layers are measured in the TTL namely, (1) moist layer (ML) with water vapour content of 4.8–5.7ppmv in altitudes of 18–19km altitudes in the lower stratosphere, and (2) ice layer (IL) with ice content up to 1.9eq.ppmv in altitudes of 17–18km in the upper troposphere around 06:30UTC on 8 August to the south of Kathmandu (Nepal). A Meso-NH convection-permitting simulation succeeds in reproducing the characteristics of ML and IL. Through analysis, we show that ML and IL are generated by convective overshoots that occurred over the Sichuan basin about 1.5 day before. Overshooting clouds develop up to 19km, hydrating the lower stratosphere of up to 20km with 6401t of water vapour by a strong-to-moderate mixing of the updraughts with the stratospheric air. A few hours after the initial overshooting phase, a hydration patch is generated, and a large amount of water vapour (above 18ppmv) still remains at even higher altitudes up to 20.5kma.s.l. while the anvil cloud top descends to 18.5km. At the same time, a great part of the hydrometeors falls shortly, and the rest sublimates. Meanwhile ice sediments out, the water vapour concentration in ML and IL decreases due to turbulent diffusion by mixing with the tropospheric air. As the hydration patch continues to travel toward the south of Kathmandu, tropospheric tracer concentration increases up to ~30 and 70% in ML and IL, respectively. The air mass in the layers becomes gradually diffused and it has less and less water vapour and ice content by mixing with the dry tropospheric air.
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Submitted on : Tuesday, November 20, 2018 - 5:49:00 PM
Last modification on : Wednesday, May 15, 2019 - 3:49:36 AM

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Keun-Ok Lee, Thibaut Dauhut, Jean-Pierre Chaboureau, Sergey Khaykin, Martina Krämer, et al.. Convective hydration in the tropical tropopause layer during the StratoClim aircraft campaign: Pathway of an observed hydration patch. Atmospheric Chemistry and Physics Discussions, European Geosciences Union, 2018, pp.(Under Review). ⟨10.5194/acp-2018-1114⟩. ⟨insu-01928820⟩

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