Sensitivity of FORUM/EE9 to Trace Gases and Surface Properties in Clear Sky

The FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission concept has been selected as one of the two candidates missions for the ESA's Earth Explorer 9 mission. FORUM will fly on a polar orbit with a ground footprint of 15 km diameter. The payload of FORUM is a Fourier Transform Interferometer (FTI) and an imager to be used to identify uniform scenes. The primary goal of the FORUM mission is the characterization of the Outgoing Longwave Radiation in the far-infrared and in the middle infrared regions, from 100 to 1600 cm-1. However, the measurement of the spectrally resolved emission of the Earth in that spectral region may be exploited to measure ice clouds, the vertical distribution of several trace gases (i.e. water, ozone) and the surface properties, especially over the polar regions where the relatively dry conditions enable to directly observe the Earth surface in the far-infrared. Moreover, the relatively high spectral resolution of FORUM will help in improving both the spectroscopy and the continuum knowledge in this so far underexploited spectral region. FORUM is currently undergoing phase A and several studies are ongoing to address the capability of the proposed instrument to provide information on important climate variables like surface temperature and emissivity, vertical distributions of humidity, temperature, trace gases, and ice clouds. Among them the FORUM-Req project is devoted to the consolidation of the instrumental requirements for the FTI and the imager in order to ensure that the science goals of FORUM will be met by the developed payload. In this paper we summarize the results obtained in the frame of the FORUM-Req project with the simulations of FORUM measurements in clear sky conditions. A set of representative atmospheric and surface conditions have been selected and used to simulate the radiation that FORUM will measure in its lifetime. These simulations have been used to study the accuracy with which FORUM will measure the UTLS distribution of trace gases and the polar surface properties and the sensitivity of the obtained results to systematic biases.