FORUM EE9 Mission: Characterization of Clear-Sky Products and Synergies With IASI-NG

Idealized model simulations suggest that between one quarter and one third of the total clear-sky long-wave cooling of the Earth to space occurs within the Far InfraRed (FIR) spectral region. An even larger fraction of this cooling is predicted to occur in the FIR under all-sky conditions, because the presence of clouds causes lower emitting temperatures, and hence a shift to longer wavelengths of the peak of the black-body emission. For this reason, a detailed study of the Earth's radiation budget requires accurate, frequent, and global-coverage measurements of the total upwelling radiance. In November 2017 the European Space Agency (ESA) selected FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) as one of the two instrument concepts to be developed further and to compete to be the ninth Earth Explorer mission. FORUM will be a Far-Infrared Spectrometer measuring the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum from polar satellite. In particular, the instrument will cover the spectral range from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), covering the FIR, including the 100 - 667 cm-1 band, which has never been spectrally resolved from space before. The main scientific objectives of FORUM include: o Disentanglement and characterization of several climate forcing variables that contribute to the Outgoing Longwave Radiation (OLR) flux in the FIR spectral region, with a particular focus on water content in the Upper-Troposphere Lower-Stratosphere (UTLS) range, and cirrus clouds. o Improvements in cloud models: need to achieve consistency of models in the Far- and Mid- InfraRed parts of the spectrum, retrieval of cloud micro-physical properties. o Improvement of spectroscopy, with a particular focus on: achievement of inter-consistency between spectral band intensities in the Middle InfraRed (MIR) and FIR regions. Improvement of atmospheric continuum models in the FIR. o Spectrally-resolved measurement of the emissivity in the FIR, of ice/snow in Polar regions. As part of ESA Phase A activities, several studies are being carried-out to consolidate the FORUM requirements needed to meet the above scientific objectives. In this work we show how FORUM measurements, thanks to their high radiometric accuracy, will permit to obtain an accurate estimate of the upwelling FIR integrated radiance. In addition, the spectral resolution of the measurements will enable to study the individual contributions driving the outgoing long-wave flux: the atmospheric composition versus height, clouds and surface type and temperature. Furthermore, FORUM mission will fly in tandem with the MetOp satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). Since IASI-NG will measure only the MIR part of the upwelling atmospheric spectrum (from 645 to 2760 cm-1), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements which are less sensitive to the water vapor content in the UTLS, an important driver of the OLR flux. In this work results of (synthetic) synergistic FORUM and IASI-NG retrievals will be presented, along with an assessment of the information gained when FORUM measurements are available in addition to those of IASI-NG.