Estimation of tropospheric water vapor using differential attenuation at microwaves and comparison with other measurements

Measuring water vapor (WV) in the troposphere, where nearly all atmospheric WV is concentrated, is critical for understanding atmospheric composition and dynamics comprehensively. A particularly challenging issue is conducting systematic WV measurements in the lower troposphere (approximately 5–6 km) on a global scale, as this would significantly enhance both climate modeling and numerical weather prediction (NWP) capabilities over short time scales.

Based on theoretical studies conducted for the European Space Agency (ESA), some of the authors proposed an innovative approach - the Normalized Differential Spectral Attenuation (NDSA) - capable of retrieving integrated water vapor (IWV) from attenuation measurements taken in the 17–21 GHz frequency band along microwave links crossing the troposphere. The NDSA technique relies on estimating a parameter, called spectral sensitivity (S), which quantifies the differential attenuation experienced by a pair of tone signals separated by a fractional bandwidth of less than 2%. It has been demonstrated that S can be directly converted into IWV using a linear relationship. Through the aforementioned ESA studies, the authors have also shown that the NDSA method can successfully estimate WV vapor from space by utilizing sets of co-rotating or counter-rotating Low Earth Orbit (LEO) satellites.

Recently, the Italian Space Agency supported the SATCROSS project, aimed at demonstrating the feasibility of a future space mission and to develop a prototype for NDSA measurements along terrestrial links operating at 19 GHz. A critical step toward consolidating progress and advancing the realization of a space-based measurement project using the NDSA approach is the performance analysis of the IWV estimates provided by prototype instruments. This includes validating those estimates by comparing them with results from other sensors and techniques, which was the objective of a four-month measurement campaign conducted from July to November 2024.

In this work, we present the main results of the campaign, conducted in a ground-to-ground configuration, designed to compare IWV measurements from the NDSA prototype instrument with those obtained using the Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique. MAX-DOAS retrieves IWV in the visible spectral range, specifically at about 445 nm. The independent optical device used in this study was configured to observe the same air volume as the NDSA instrument. Measurements were made along a link connecting the meteorological station "Giorgio Fea," located at the rural site of St. Pietro Capofiume, Bologna, 10 m above sea level, to the WMO/GAW (World Meteorological Organization/Global Atmosphere Watch) Climate Observatory “Ottavio Vittori” at Mount Cimone, 2165 m above sea level.

The link length is 91 km, with no physical obstacles interposed. In addition to MAX-DOAS data, measurements were also acquired and processed from radiosondes, hygrometers, GNSS (Global Navigation Satellite Systems) and a tethered balloon.

The research activities presented in this work were carried out with contribution of the Next Generation EU funds within the National Recovery and Resilience Plan (PNRR), Mission 4-Education and Research, Component C2-From Research to Business (M4C2), Investment Line 1.1-Fund for the National research program and projects of significant national interest (PRIN), Project 2022JJJYTE -``Measuring tropospheric water vapor through the Normalized Differential Spectral Attenuation (NDSA) technique''