Altitude-resolved measurements of water vapor from ground to space: the Swiss H2O-Hub 

The abundance of water vapor (H2O) in the upper troposphere-lower stratosphere (UTLS) plays a critical role for the Earth's radiative balance. Yet, accurate measurements of H2O in the UTLS are still very demanding, and lack sufficient spatial and temporal coverage. Therefore, frequent and long-term measurements with high precision and vertical resolution, as provided by balloon-borne instruments, are required. Furthermore, large discrepancies were often found between different measuring techniques, both in the field and in laboratory settings, indicating the difficulty of reliable H2O measurements at the low concentrations of the UTLS. Within the GCOS-funded Swiss H2O-Hub project, we aim to address this challenge by continuously optimizing and validating the performances of two novel balloon-borne hygrometers, while exploiting their complementarity and vertical overlap with remote sensing techniques. Particularly, the balloon-borne laser spectrometer for UTLS water research ("ALBATROSS") [1,2], based on mid-IR laser absorption spectroscopy, and the Peltier-cooled frostpoint hygrometer (PCFH) [3], based on the chilled-mirror principle, are operated jointly with remote sensing measurements by the Raman lidar for meteorological observations (RALMO) [4] and the middle atmospheric water vapor radiometer (MIAWARA) [5]. The altitude coverages of RALMO (troposphere) and MIAWARA (stratosphere and mesosphere) offer two valuable overlap regions for the UTLS measurements by ALBATROSS and PCFH, while all together this set of instruments provides the unique possibility to monitor the altitude-resolved H2O profile from ground to space. Here we report on the results of the first measurement campaign of the project, conducted in summer 2023 at the MeteoSwiss Observatory Payerne. This included in total 7 balloon soundings with PCFH and ALBATROSS, along with simultaneous retrievals by RALMO and MIAWARA. All balloon-borne payloads were accompanied by a Vaisala RS41 radiosonde and a cryogenic frostpoint hygrometer (CFH) instrument as a reference. The data show very good agreement between the different techniques in the upper troposphere, and some limitations in the lower stratosphere. This is a promising result in the context of the ongoing reconception of the CFH method owing to its use of fluoroform (HFC-23) as cooling agent, which must be phased out due to its high global warming potential. Additionally, the MIAWARA measurements revealed the signature of the Hunga Tonga-Hunga Ha'apai volcanic eruption on H2O in the upper stratosphere and mesosphere. Further measurement campaigns, planned for the upcoming years, will allow to refine the performances of all instruments under UTLS conditions, as well as to continue monitoring the interannual variability and trends in upper air H2O over Switzerland. [1] Graf et al., Atmos. Meas. Tech., 14, 1365–1378, 2021. [2] Brunamonti et al., Atmos. Meas. Tech., 16, 4391–4407, 2023. [3] Jorge, Diss. ETH No. 26352, 2019. [4] Dinoev et al., Atmos. Meas. Tech., 6, 1329–1346, 2013. [5] Straub et al., Atmos. Meas. Tech., 3, 1271–1285, 2010.