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. The source and pathway of the hydration patch in the TTL (tropicaltropopause layer) that was measured during the Stratospheric and upper troposphericprocesses for better climate predictions (StratoClim) field campaignduring the Asian summer monsoon in 2017 and its connection to convectiveovershoots are investigated. During flight no. 7, two remarkable layers aremeasured in the TTL, namely (1) the moist layer (ML) with a water vapour contentof 4.8–5.7 ppmv in altitudes of 18–19 km in the lowerstratosphere and (2) the ice layer (IL) with ice content up to 1.9 eq. ppmv (equivalent parts per million by volume) inaltitudes of 17–18 km in the upper troposphere at around 06:30 UTC on 8 August to the south of Kathmandu (Nepal). A Meso-NH convection-permittingsimulation succeeds in reproducing the characteristics of the ML and IL. Throughanalysis, we show that the ML and IL are generated by convective overshoots thatoccurred over the Sichuan Basin about 1.5 d before. Overshooting cloudsdevelop at altitudes up to 19 km, hydrating the lower stratosphere of up to 20 km with6401 t of water vapour by a strong-to-moderate mixing of the updraughts withthe stratospheric air. A few hours after the initial overshooting phase, ahydration patch is generated, and a large amount of water vapour (above 18 ppmv) remains at even higher altitudes up to 20.5 km while the anvil cloudtop descends to 18.5 km. At the same time, a great part of the hydrometeorsfalls shortly, and the water vapour concentration in the ML and IL decreases dueto turbulent diffusion by mixing with the tropospheric air, ice nucleation,and water vapour deposition. As the hydration patch continues to traveltoward the south of Kathmandu, tropospheric tracer concentration increasesup to ∼30 % and 70 % in the ML and IL, respectively. The airmass in the layers becomes gradually diffused, and it has less and less watervapour and ice content by mixing with the dry tropospheric air.

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