FORUM: A Mission to Measure the Earth's Far Infrared Outgoing Radiation Spectrum in order to improve our ability to Monitor and Predict Climate.

It has long been acknowledged that the far infrared (far-ir: defined here as wavelengths > 15 ?m) plays a pivotal role in determining our planet's energy budget. Under clear-sky conditions the region contributes of the order a quarter to one-third of the Earth's Greenhouse Effect. Under all-sky conditions its influence on the Earth's emitted energy is even larger, with the typically colder emitting temperatures of mid and high level cloud shifting the peak of the Planck function to longer wavelengths such that, in the global mean, approximately half of the Earth's emission to space occurs within the far-ir. The dominant role of the far-ir in determining the Earth's Outgoing Longwave Radiation (OLR) is in part due to the strong water vapour rotation band at wavelengths > 16.5 ?m. This in turn means that radiative emission in the far-ir is particularly sensitive to water vapour in the climatically important upper troposphere/lower stratosphere (UTLS) region. Similarly, clear-sky longwave radiative cooling through the mid and upper troposphere is dominated by the contribution from the far-ir. While for much of the globe this water vapour absorption means that far-ir surface emission cannot be sensed from space, in very dry, clear-sky conditions experienced in the Arctic and Antarctica this is no longer the case. Here the outgoing longwave radiation becomes sensitive to surface emission within the far-ir through a number of micro-windows which become progressively more transmissive as water vapour concentration reduces. Very recent studies show that this previously unexplored phenomenon may play a critical role in both reducing persistent climate model biases and determining the pace of Arctic and Antarctic climate change. Moreover, cirrus clouds, typically poorly constrained by observations yet crucial players in determining current and future climate, have emitting temperatures that place the peak of their radiative emission within the far-ir. Our ability to correctly simulate the interaction of the radiation spectrum with cirri relies on the capability of optical models to adequately represent their scattering and absorption properties. These are crucially dependent on the complex ice-crystal shapes and their size distributions within the clouds. Recent advances in cirrus cloud optical modelling have attempted to capture the bulk microphysical properties of cirri spanning the entire electromagnetic spectrum. However, while there are many satellite observations of the reflected visible and emitted near- and mid-infrared radiation in the presence of cirrus clouds that can be exploited to test these developments, there are no such observations that span the far-ir. This represents a major barrier to improving our confidence in our ability to understand and monitor cirrus properties and their interaction with the Earth's outgoing longwave energy, particularly since the contrast in ice and water refractive indices between the far-ir and mid-infrared implies that unique information relating to cloud classification and microphysics can be leveraged from measurements of the far-ir spectrum. Despite the sensitivity of the far-ir to these key climate drivers, to date there has been no dedicated satellite mission to observe the complete, radiatively important far-ir spectrum. Those space-based data which do extend into the far-ir, from the InfraRed Interferometer Spectrometer (IRIS-D) mission, only reach 25 ?m and are limited in temporal extent from April 1970 to February 1971. Moreover, there are questions regarding their suitability for quantitative climate related studies. From a purely energetics viewpoint the absence of a dedicated far-ir mission constitutes the major missing piece of our climate observing system. Recognising this, the European Space Agency (ESA) has selected the Far infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission as one of two candidates for their Earth Explorer 9 programme. In this talk we will discuss the scientific motivation for the mission, presenting results from a number of studies which have helped to shape the design concept. In particular, we will demonstrate how FORUM will advance Earth system science by observing, for the first time, with high accuracy, the far-infrared spectrum of the Earth, a region which plays a fundamental role in determining our planet's energy balance.