The FORUM EE9 Proposed Mission: Potentiality of Measurements and Synergies with IASI-NG

In November 2017 the European Space Agency 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 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 Far InfraRed (FIR), including the 100 and 667 cm -1 band, which has never been spectrally resolved from space before. 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 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 curve. For this reason, a detailed study of the Earth's radiation budget requires accurate, frequent, and global-coverage measurements of the total upwelling irradiance. In this work we will show how FORUM measurements will permit to obtain a very accurate estimate of the upwelling FIR irradiance. Furthermore, the spectral resolution of the measurements will enable to disentangle the contributions of the various contributions (due to gases, surface, clouds) to the outgoing long-wave flux. Since the FIR part of the upwelling spectral radiance is particularly sensitive to the water vapor content in the Upper Troposphere, FORUM measurements will effectively complement, by flying in tandem with MetOp, the Middle-InfraRed spectrum that will be measured by IASI-NG. In this paper we will show how FORUM and IASI-NG measurements could be synergistically exploited to improve the present knowledge of the water vapor amount in the Upper Troposphere.