Comment on amt-2022-13

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 differences between ChiWIS and FLASH were only −1.42 % and those between FISH and FLASH only −1.47 %. Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at +0.74 %. In general, ChiWIS and FLASH agreed to better than 10 % for 92 % (87 %) of clear-sky (in-cloud) datapoints. Agreement between FISH and FLASH to 10 % occurred in 78 % of clear-sky datapoints. Estimated realized instrumental precision in UTLS conditions was 0.05, 0.1, and 0.2 ppmv for ChiWIS, FISH, and FLASH, 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.