The FORUM-EE9 mission: potentiality of the FIR measurements of the Outgoing Longwave Radiation for the estimate of geophysical variables

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been selected to be the Earth-Explorer 9 mission of the European Space Agency. The main instrument on board of FORUM is a Fourier Transform Interferometer measuring the outgoing longwave radiation from 100 to 1600 cm-1, designed to provide for the first time the spectrally resolved full emission spectrum of the Earth and the atmosphere. The far infrared (FIR) region below 600 cm-1 contains large part of the information on climatological key variables strongly driving the Earth Radiation Budget. This work investigates the information content of the FORUM synthetic radiances in a large set of atmospheric scenarios. First, the sensitivity of the acquired signal in the FIR is studied by analysing the derivatives of the spectrum with respect to water vapour concentration and surface emissivity. The precision on these variables is estimated from the inversion of synthetic FORUM measurements. We then have calculated the systematic contribution due to different systematic sources on these parameters. The uncertainties on water vapour spectroscopic data is a relevant source of systematic error, therefore we assessed the consistency of recent spectroscopic data by analysing the agreement between the simulations performed using different spectroscopic databases and the REFIR-PAD measurements over Teresina (Brazil). Finally, the airborne measurements of the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) have been analysed to retrieve water vapour VMR from 0 to 9 km. The quality of the obtained products has been estimated studying the consistency of the retrieved profiles with those evaluated by in-situ dropsondes and the Numerical Weather Prediction model, determining also the information gain on water vapour VMR in the upper troposphere due to the measurements in the FIR range.