A thorough scientific understanding of the spectral distribution of the infrared energy emitted by the Earth system is of the utmost importance, being fundamentally linked to the Earth’s Radiation Budget (ERB), and implicitly containing signatures of many of the components responsible for driving, and responding to, climate change.
In the global mean, approximately half of the Earth’s outgoing longwave radiation (OLR) to space is located at wavelengths greater than 15 ?m, within the so-called far infrared (FIR). This wavelength region is highly sensitive to upper tropospheric water vapour and to cirrus cloud, both of which critically influence the ERB and climate sensitivity. Moreover, the FIR has a more important role than previously recognised in determining the pace of change in our fragile polar regions. Nevertheless, despite its importance, the FIR has never been measured spectrally, in its entirety, from space, due primarily to the technical difficulties associated with achieving the necessary instrument signal to noise across the region. The situation has now changed thanks to recent advances that have been made in both detector and optical technology.
The Far-infrared-Outgoing-Radiation Understanding and Monitoring (FORUM) mission will build on these developments, and will, as its main goal, fill the long-standing gap in FIR spectral observations. Previous space missions that sampled part of the FIR (extending to wavelengths of ~25 ?m) were exploratory in nature and had neither the necessary lifetime nor accuracy to provide quantitative measurements at the level required for climate change monitoring. By closing this gap, FORUM will deliver an improved understanding of the climate system, informing climate policy decisions by supplying, for the first time, a complete characterisation of the Earth’s OLR spectrum.
This goal will be achieved by performing a spectral measurement that:
- covers the Earth’s top-of-atmosphere (TOA) emission spectrum from 100 to 1600 cm-1 (100–6.25 ?m) with a spectral resolution of 0.5 cm-1 (defined as full width at half maximum of the instrumental spectral response function);
- fills the observational gap across the far-infrared (from 100 to 667 cm-1), never before sounded in its entirety from space;
- complements, by flying in tandem with the Meteorological Operational Satellite – Second Generation (Metop-SG), mid-infrared spectral measurements performed in the 645–2760 cm-1 range by the Infrared Atmospheric Sounding Instrument Next Generation (IASI-NG);
- provides a three-years dataset benchmarked against international standards with an absolute radiometric accuracy (ARA) of at least 0.1 K in the spectral range 300-1100 cm-1 (3σ for scene temperatures between 190-300 K). ARA will be 0.2K in the ranges 200-300 cm-1 and 1100-1300 cm-1.
In this way, FORUM together with IASI-NG will deliver a truly unique dataset of the Earth’s entire emission spectrum from 100 to 2760 cm-1 (3.62–100 ?m) which can be used independently to probe FIR energetics but will also efficiently complement existing and planned future missions (e.g. RBI, Sentinel 4/IRS, etc.). It will provide a stringent test of our understanding of, and ability to model, the links between key underlying physical processes driving climate change, their spectral signatures, the Greenhouse Effect, and the overall ERB.
While the delivery of a climate quality radiance dataset spanning the FIR represents a major scientific outcome in its own right, additional mission objectives include:
- derivation of the FIR optical properties of cirrus clouds;
- measurement of FIR surface emissivity;
- measurement of UTLS H2O and associated FIR spectral signatures;
- derivation of FIR spectral fluxes (spectral greenhouse effect, spectral clo
ud forcing, climate model diagnostics);
- measurement of FIR radiance “benchmark”;
- improvement in water vapour and carbon dioxide spectroscopy;
- reference intercalibration of broadband ERB sensors;
To meet these objectives, the observing mode should be nadir-viewing with a threshold alongtrack sampling of 100 km, comparable to the spatial resolution of current global general circulation models. The proposed mission lifetime is 3 years in order to perform measurements covering different seasons and capture inter-annual variability. A flight configuration in tandem with IASI-NG requires a sun-synchronous orbit at an altitude of about 817 km, as anticipated for Metop-SG. The fixed overpass time precludes studies of diurnal variability, but provides a consistent set of observations for climatological assessments. The ground footprint is a single pixel of about 15 km comparable to IASI-NG observations.