Innovative and State-of-the-Art Remote Sensing techniques for Tropospheric Water Vapor Estimation: Insights from a Po Valley Campaign

A comprehensive understanding of atmospheric composition and dynamics requires a detailed knowledge of water vapour (WV). In particular, characterizing WV in the lower troposphere is essential for enhancing the performance of climate modeling and numerical weather prediction (NWP), while also posing a significant challenge. Remote sensing techniques enable the investigation of WV behavior by leveraging the capabilities of different spectral ranges, from visible to microwave. While ground-based GNSS (Global Navigation Satellite Systems) measurements can be used to provide total column WV, the Normalized Differential Spectral Attenuation (NDSA) is an innovative approach capable of retrieving integrated water vapor (IWV) from attenuation measurements taken in the 17–21 GHz frequency range along a link that connects a transmitter with a receiving antenna. Ground-based DOAS and MAX-DOAS observations play a crucial role in monitoring trace gases in the troposphere and have also been used in the past to retrieve WV. As part of the Italian PRIN-NDSA project, a measurement campaign was carried out in the Po Valley from July to November 2024, focusing on tropospheric IWV. The campaign utilized a ground-to-ground configuration of the NDSA prototype, with a link that connected the meteorological station "Giorgio Fea" in San Pietro Capofiume (10 m a.s.l.), Bologna, to the WMO/GAW (World Meteorological Organization/Global Atmosphere Watch) Climate Observatory “Ottavio Vittori” at Mount Cimone (2165 m a.s.l.). Dedicated GNSS total column WV measurements were performed at San Pietro Capofiume, while WV profiles and corresponding integrated values were obtained from daily radiosoundings. Due to the relevance of the Po Valley for atmospheric and air quality studies, the San Pietro Capofiume observatory has been equipped with a SkySpec-2D MAX-DOAS instrument since October 2021. The SkySpec-2D’s capability to measure both at Zenith and off-axis, as well as to retrieve WV Slant Column Densities (SCDs), offers a unique opportunity to investigate IWV along different viewing directions. This work presents the results obtained during the campaign, focusing on MAX-DOAS IWV retrievals and their comparison with results from NDSA, GNSS, and radiosoundings.