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

驱动气候变化的因素是什么？我们如何限制对未来气候系统如何演变的预测？研究地球的辐射收支，特别是它发出的长波能谱，为回答这些问题提供了一条有希望的途径。理论模型表明，全球范围内，地球发射回太空的辐射中有一半的波长大于15微米。这一光谱区域被称为远红外，受到水蒸气、云以及高纬度地区地表特性的强烈影响：所有这些都与关键的气候反馈有关。然而，地球的远红外光谱从未被测量过。这一令人震惊的疏忽将在未来几年发生改变。NASA将发射极地辐射能量远红外实验（PREFIRE），这是一个专注于极地地区的小型立方体卫星任务，而一个更全面的任务，FORUM（远红外发射辐射理解和监测）（https://www.forum-ee9.eu/）最近被选为欧洲航天局的第九个“地球探测任务”。由于FORUM计划于2026/27年发射，因此该项目是参与卫星任务开发的难得机会。虽然这个项目的确切方向是非常开放的，一个感兴趣的领域涉及卷云-高冰云-在气候系统中的作用。它们特别有趣，因为与其他类型的云相比，它们被假设对气候有显著的变暖影响，其中大部分预测在远红外中实现。FORUM将允许我们研究这些云的微物理，并使用RAL开发的最佳估计方案，从其测量中检索云的属性。要成功做到这一点，需要对检索中使用的卷云光学特性模型进行测试，这是可能在卫星发射之前使用帝国理工学院对流层机载傅立叶变换光谱仪（TAFTS）的独特飞机观测结果进行测试的。其他感兴趣的领域包括利用TAFTS数据进行表面性质和水蒸气检索。此外，欧空局还委托帝国理工大学空间和大气物理小组的研究人员为FORUM开发一种新的基于飞机的仪器演示器。因此，将有空间帮助塑造它的设计和建造，并可能参与科学试飞。

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.