Water vapor variability in the extratropical UTLS from combined passenger and reasearch aircraft measurements

Water vapor (H₂O) is a key trace gas in the upper troposphere (UT) and lowermost stratosphere (LMS) because it plays a crucial role in the Earth’s climate. However, accurate knowledge of the amount of H₂O in this region is still insufficient due to the difficulty and lack of in-situ and space-borne measurements. This study presents a new methodology to compile H₂O climatologies for the LMS from simple, extensive measurements aboard passenger aircraft between 1994 and now within the IAGOS infrastructure covering in the extratropical UT/LMS.

To this end, a statistical comparison of mean H₂O in sampling bins of air relative to the tropopause is performed between a dataset from ≈60.000 flights applying the IAGOS-MOZAIC and -CORE simple sensor and a dataset of only ≈500 flights using the more sophisticated IAGOS-CARIBIC instrument. We find good agreement in the UT, but a systematic positive bias in the simple measurements in the LMS. To account for this bias, mean water vapor values of the simple sensor are adjusted to the sophisticated observations based on a new statistical approach. After applying this new method, the LMS water vapor measurements are in good agreement. The extensive H₂O dataset from the simple IAGOS sensor can now be used to produce highly resolved water vapor climatologies for the climatically sensitive LMS region. With the adjusted IAGOS H₂O data, water vapor transport processes and (de-)hydration of air masses in the extratropical UT/LMS are analysed through backward trajectories and microphysical CLaMS-ICE simulations.