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  1. Abstract. In situ measurements in the climatically important upper troposphere–lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, and hydration–dehydration of the tropical tropopause layer.Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (<5 ppmv) and a challenging low temperature–pressure environment.The StratoClim campaign out of Kathmandu, Nepal, in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer).Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence.In clear-sky UTLS conditions (H2O<10 ppmv), mean and standard deviations of differences in paired observations between ChiWIS and FLASH were only (-1.4±5.9) % and those between FISH and FLASH only (-1.5±8.0) %.Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at (+0.7±7.6) %.Estimated realized instrumental precision in UTLS conditions was 0.05, 0.2, and 0.1 ppmv for ChiWIS, FLASH, and FISH, respectively.This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget. 
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  2. Abstract. The Asian monsoon anticyclone (AMA) represents one of thewettest regions in the lower stratosphere (LS) and is a key contributor tothe global annual maximum in LS water vapour. While the AMA wet pool islinked with persistent convection in the region and horizontal confinementof the anticyclone, there remain ambiguities regarding the role oftropopause-overshooting convection in maintaining the regional LS watervapour maximum. This study tackles this issue using a unique set ofobservations from aboard the high-altitude M55-Geophysica aircraft deployedin Nepal in summer 2017 within the EU StratoClim project. We use acombination of airborne measurements (water vapour, ice water, waterisotopes, cloud backscatter) together with ensemble trajectory modellingcoupled with satellite observations to characterize the processescontrolling water vapour and clouds in the confined lower stratosphere (CLS)of the AMA. Our analysis puts in evidence the dual role of overshootingconvection, which may lead to hydration or dehydration depending on thesynoptic-scale tropopause temperatures in the AMA. We show that all of theobserved CLS water vapour enhancements are traceable to convective eventswithin the AMA and furthermore bear an isotopic signature of the overshootingprocess. A surprising result is that the plumes of moist air with mixingratios nearly twice the background level can persist for weeks whilstrecirculating within the anticyclone, without being subject to irreversibledehydration through ice settling. Our findings highlight the importance ofconvection and recirculation within the AMA for the transport of water into thestratosphere. 
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