Total column water vapor from Sentinel-4 and Sentinel-5 measurements: initial results

Kurzfassung

Water vapor in the atmosphere serves as a critical component in both the greenhouse effect and hydrological cycle, representing the predominant naturally occurring greenhouse gas within Earth's climate system. Its significantly greater spatiotemporal variability compared to other major greenhouse gases such as carbon dioxide and methane necessitates satellite-based global monitoring to elucidate climate impacts at both regional and global scales over short-to-multidecadeal timescales.

This study presents recent advancements in retrieving atmospheric water vapor total column measurements from satellite observations of reflected solar radiation within the UV-VIS spectral region centred at 440 nm using Differential Optical Absorption Spectroscopy (DOAS). This water vapor absorption band represents an alternative to the conventional NIR band (centred at 648 nm), with the distinct advantage of being detectable across all European Sentinel platforms, their predecessor instruments (GOME on ERS-2, SCIAMACHY on Envisat, GOME-2 on MetOp-A/B/C), and future instruments (e.g., CO2M).

The methodology is applied to observations from the geostationary Sentinel-4/UVN spectrometer aboard MTG-S1 (launched July 2025) and the polar-orbiting Sentinel-5 spectrometer aboard MetOp-SGA (launched August 2025). Sentinel-4 represents the first instrument of its kind in geostationary orbit focused on Europe, generating spectra on an hourly basis and providing unprecedented insights into diurnal variations over Europe, the Mediterranean, parts of Northern Africa, and the Atlantic regions within its field of view. This capability significantly enhances temporal sampling frequency compared to existing LEO instruments.

The retrieval methodology demonstrates several advantages over existing sensing techniques: (1) optimal sensitivity and coverage characteristics across both terrestrial and oceanic domains; (2) enhanced spatial resolution and temporal sampling frequency, particularly for European observations via Sentinel-4, improving weather forecasting, scientific product development, air quality assessment, and environmental policy applications; and (3) continuous extension of long-term datasets starting with GOME-type sensors since 1995, crucial for regional and global climate modeling and enabling the creation of a homogeneous climate data record spanning three decades.