In-situ measurements of the HDO/H2O Isotopic ratio in the Asian Summer Monsoon trace strong convective influence on cirrus

Abstract:

Understanding the amount and distribution of thin cirrus in the upper troposphere requires understanding the downstream effects of deep convection, which carries large amounts of ice to the tropopause level. Convective influence may be important for days or weeks, even after initial sublimation of detrained ice, since cirrus distributions are affected by post-convective water vapor distributions. We show the extent of downstream influence in in-situ observations from the Asian Monsoon Anticyclone (AMA) during the 2017 StratoClim campaign, a series of eight high-altitude flights in July—August that sampled a wide range of monsoon behavior, with diversity in airmass origin (Northwest India to China) and time since last convective influence (from over a week to minutes-old outflows at tropopause altitudes). In-situ measurements of the HDO/H₂O isotopic ratio from the Chicago Water Isotope Spectrometer (ChiWIS) provide a tracer of water whose origin is sublimated convective ice, which has a characteristic heavy isotopic signature. We sample frequent characteristic water vapor perturbations from convective detrainment plumes, marked by increased concentration (up to a factor of two above surroundings), links by back-trajectory calculations to areas of prior convective activity, and significant isotopic anomalies. Convective perturbations appear to persist for days to weeks and in some cases drive formation of ``secondary’’ in-situ cirrus days after sublimation. Vertical profiles of water isotopic composition demonstrate substantial convective influence up to 400 K potential temperature, in agreement with other tracers, during all phases of the campaign. While the Asian monsoon anticyclone is a unique meteorological environment, the finding of extensive production of secondary cirrus should generalize to the wider tropical upper troposphere, suggesting that future changes in tropical deep convection may have widespread indirect influence on cirrus statistics. The observation of persistent isotopic signals also suggests that convective influence will be detectable even in the aged monsoon outflow that should be sampled in the upcoming 2021 ACCLIP campaign.