Abstract. The source and pathway of the hydration patch in the TTL (tropical
tropopause layer) that was measured during the Stratospheric and upper tropospheric
processes for better climate predictions (StratoClim) field campaign
during the Asian summer monsoon in 2017 and its connection to convective
overshoots are investigated. During flight no. 7, two remarkable layers are
measured in the TTL, namely (1) the moist layer (ML) with a water vapour content
of 4.8–5.7 ppmv in altitudes of 18–19 km in the lower
stratosphere and (2) the ice layer (IL) with ice content up to 1.9 eq. ppmv (equivalent parts per million by volume) in
altitudes 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-permitting
simulation succeeds in reproducing the characteristics of the ML and IL. Through
analysis, we show that the ML and IL are generated by convective overshoots that
occurred over the Sichuan Basin about 1.5 d before. Overshooting clouds
develop at altitudes up to 19 km, hydrating the lower stratosphere of up to 20 km with
6401 t 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 18 ppmv) remains at even higher altitudes up to 20.5 km while the anvil cloud
top descends to 18.5 km. At the same time, a great part of the hydrometeors
falls shortly, and the water vapour concentration in the ML and IL decreases due
to turbulent diffusion by mixing with the tropospheric air, ice nucleation,
and water vapour deposition. As the hydration patch continues to travel
toward the south of Kathmandu, tropospheric tracer concentration increases
up to ∼30 % and 70 % in the 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.
Convective hydration in the tropical tropopause layer during the StratoClim aircraft campaign: pathway of an observed hydration patch
