Technical note: Water vapour climatologies in the extra-tropical upper troposphere and lower stratosphere derived from a synthesis of passenger and research aircraft measurements

Abstract. This study presents a new methodology to derive adjusted water vapour (H2O) climatologies for the extra-tropical upper troposphere and lowermost stratosphere (UT/LMS) from regular measurements on board passenger aircraft between 1994 and 2022 within the IAGOS (In-service Aircraft for a Global Observing System) research infrastructure. A synthesis of mean H2O is performed by sampling air mass bins of similar origin and thermodynamic conditions relative to the tropopause between a dataset from 60 000 flights employing the IAGOS-MOZAIC (Measurement of Ozone by AIRBUS In-Service Aircraft) and IAGOS-CORE capacitive hygrometer (ICH) and a dataset of 500 flights using the more sophisticated IAGOS-CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container) hygrometer. The analysis is, in combination with ECMWF ERA5 meteorological data, accomplished for the extra-tropical Northern Hemisphere, where the datasets have the largest common coverage. We find very good agreement in the UT but a systematic positive humidity bias in the ICH measurements for the LMS. To account for this bias, mean H2O of the ICH is adjusted to the IAGOS-CARIBIC measurements based on a new mapping and adjustment approach. After applying this new method, the LMS H2O measurements are in good agreement between all investigated platforms. The extensive H2O dataset from the compact IAGOS sensor can now be used to produce highly resolved H2O climatologies for the climatically sensitive LMS region.