The dual role of Asian deep convection in cross-tropopause transport of water: hydration and dehydration

The Asian Monsoon Anticyclone (AMA) represents the wettest region in the lower stratosphere (LS) and is a key contributor to the global annual maximum in LS water vapor. While the AMA wet pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, the impact of overshooting convection on AMA water vapor is not well understood. This study addresses this issue using a unique set of observations onboard high-altitude M55-Geophysica aircraft deployed in Kathmandu in Summer 2017 as part of the EU StratoClim project. The study incorporates airborne measurements of water vapor, ice water, water isotopes, carbon monoxide, ozone as well as cloud particle backscatter and size distributions; ensemble trajectory modeling coupled with a satellite-based overshoot tracking scheme; as well as an extensive set of satellite observations.

The analysis provides a detailed insight into a complex variety of processes driving water distribution in the Asian tropopause layer. In particular, we show evidence of direct and indirect convective impact on LS water up to the 415 K level. The ensemble trajectory analysis suggests that tropopause-overshooting convection above subtropical China may be a key contributor to AMA hydration, while deep convection above Northern India counteracts the moistening through convectively-induced cooling of the tropopause. With that, both hydration and dehydration processes in AMA may be reversible, resulting in a strong variability of H₂O above the climatological tropopause.

The intensity of convective influence is demonstrated in the flight of 10 Aug 2017, in which the aircraft flew through active stratospheric overshoots at the cold-point tropopause and sampled outflows minutes to hours old. The measurements reveal up to 2500 ppmv of ice water above 17 km in large aggregated ice crystals up to 700 μm in diameter. Smaller crystals were observed as high as 18.8 km (410 K). Tracer measurements show perturbations reflecting vigorous vertical motions and provide evidence for ongoing mixing of tropospheric and stratospheric air around the tropopause. The ensemble of measurements inside and downwind of convective overshoots is used to characterize in detail the impact of overshooting convection on the structure and composition of the Asian tropopause layer.