The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) Mission

Abstract:

The Far-infrared Outgoing Radiation Understandingand Monitoring (FORUM) mission has recently been selected as ESA’s 9^th Earth Explorer mission to be launched in 2026. FORUM will be the first mission to observe globally the Earth’s emitted outgoing longwave radiation (OLR) spectrum across the far-infrared (FIR), defined here as the spectral range from 100 to 667 cm^-1 (or, correspondingly, from 100 to 15 µm of wavelength) with high spectral resolution and accuracy from space.

The mission payload includes two instruments:

• a sounder, made by a Fourier transform spectrometer as the primary instrument, measuring the OLR spectrum in 100-1600 cm^-1 (100-6.25 µm) with 0.5 cm^-1 of FWHM resolution in a single footprint of 15 km of diameter and an absolute radiometric accuracy, benchmarked against international standards, of 0.1 K at 3σ;

• a co-aligned thermal imager in the infrared atmospheric window at 10.5 μm with a ground footprint of 36 x 36 km² to identify clouds and sub-pixel heterogeneities in the scenes observed by the sounder.

FORUM is planned to fly in loose formation with the Meteorological Operational Satellite - Second Generation (Metop-SG) to take advantage of the mid-infrared spectral measurements performed by the Infrared Atmospheric Sounding Instrument Next Generation (IASI-NG). In such a way, the mission will deliver for the first time the complete spectral characterisation of the OLR over almost the full range of emission, from 100 to 2760 cm^-1 (3.62-100 µm). Beside this, mission objectives include the generation of the geophysical products, such as OLR spectral fluxes, vertical profiles of water vapor concentration and temperature; cirrus cloud properties (top/base height, ice water path; effective diameter of particles); surface emissivity, with a focus on the new FIR spectral emissivity of frozen surfaces in polar regions.

These measurements are expected to provide an accurate basis for the detection of the spectroscopic climate fingerprints (due to water vapour, clouds, methane, carbon dioxide, and snow/ice surface emission) that will significantly enhance our understanding of key forcing and feedback processes of the Earth's atmosphere, enabling more stringent evaluation of climate models.

In this presentation, an overview of the FORUM mission and the current development status will be presented.