Water Vapor climatologies in the extra-tropical Upper Troposphere and Lower Stratosphere derived from a Synthesis of Passenger and Research Aircraft Measurements

Abstract. Water vapor (H₂O) is a key trace gas in the upper troposphere (UT) and lowermost stratosphere (LMS), as it significantly influences the Earth’s climate system through its roles in radiative forcing and cloud formation. However, accurate knowledge of the amount of H₂O in this atmospheric region is still insufficient due to the difficulty and lack of precise in-situ and space-borne measurements. This study presents a new methodology to derive adjusted H₂O climatologies for the extra-tropical UT/LMS from regular measurements aboard passenger aircraft between 1994 and 2022 within the IAGOS (In-service Aircraft for a Global Observing System) research infrastructure. To this end, 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 applying the IAGOS-MOZAIC and -CORE compact hygrometer (ICH) and a data set of 500 flights using the more sophisticated IAGOS-CARIBIC hygrometer. The analysis is, in combination with ECMWF ERA5 meteorological data, accomplished for the extratropical 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 are 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.