Understanding how Earth is losing its cool: A study in support of the FORUM satellite mission

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What drives climate change and how can we constrain predictions of how the climate system will evolve in the future? Studying the Earth's Radiation Budget and, in particular, its outgoing longwave energy spectrum, offers a promising route to answering these questions. Theoretical models suggest that globally half the radiation emitted by Earth back to space is at wavelengths greater than 15micrometers. This spectral region is referred to the far infrared and is strongly influenced by water vapour, cloud and, at high latitudes, surface properties: all of which are linked to key climate feedbacks. However, the Earth's outgoing spectrum in the far infrared has never been measured. This rather shocking oversight will change in the coming years. NASA will launch the Polar Radiant Energy in the Far InfraRed Experiment (PREFIRE) - a small cubesat mission focused on the polar regions - while a more comprehensive mission, FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) (https://www.forum-ee9.eu/) was recently selected to be the European Space Agency's ninth 'Earth Explorer Mission'. Since FORUM is scheduled for launch in the 2026/27 timeframe, this project represents a rare opportunity to be involved in the development of a satellite mission. Although the exact direction of this project is very open, one area of interest concerns the role of cirrus - high ice cloud - in the climate system. They are particularly interesting as, in contrast to other types of cloud, they are hypothesized to have a significant warming impact on climate, much of which is predicted to be realised in the far infrared. FORUM will allow us to study the microphysics of these clouds and, using optimal estimation schemes developed at RAL, retrieve cloud properties from its measurements. To do this successfully requires the cirrus optical property models used in the retrievals to be tested, something that is possible in advance of the satellite launch using unique aircraft observations from Imperial's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS). Other areas of interest include exploiting TAFTS data for surface property and water vapour retrieval. Moreover, researchers within the Space and Atmospheric Physics group at Imperial have been tasked by ESA to develop a new aircraft-based instrument demonstrator for FORUM. As such there will be scope to help shape its design and build and, possibly, participate in scientific test flights.