Abstract. Spectral emissivity is a key property of the Earth surface of which only very few measurements exist so far in the far-infrared (FIR) spectral region, even though recent work has shown its FIR contribution is important for accurate modelling of global climate. The European Space Agency's 9th Earth Explorer, FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) will provide the first global spectrally resolved measurements of the Earth's top-of-the-atmosphere (TOA) spectrum in the FIR. In clear-sky conditions with low water vapour content, these measurements will provide a unique opportunity to retrieve spectrally resolved FIR surface emissivity. In preparation for the FORUM mission with an expected launch in 2026, this study takes the first steps towards the development of an operational emissivity retrieval for FORUM by investigating the sensitivity of the emissivity product of a full spectrum optimal estimation retrieval method to different physical and operational parameters. The tool used for the sensitivity tests is the FORUM mission's end-to-end simulator. These tests show that spectral emissivity of most surface types can be retrieved for dry scenes in the 350–600 cm⁻¹ region with an uncertainty ranging from 0.005 to 0.01. In addition, the quality of retrieval is quantified with respect to the precipitable water vapour content of the scene, and the uncertainty caused by the correlation of emissivity with surface temperature is investigated. Two main recommendations are made based on these investigations: (1) As the extent of TOA sensitivity to the surface in the FIR depends on the atmospheric state, the spectral region of the emissivity product should be decided using a so-called information quantifier, calculated from the ratio of the retrieval uncertainty to the a-priori uncertainty. (2) Depending on retrieval input parameters, the correlation of emissivity with surface temperature allows for retrieved emissivities within a small range around the true emissivity. Thus the impact of this correlation on the uncertainty estimates of the product should be quantified in detail during further development of the operational retrieval.

中文翻译：

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使用 FORUM 的端到端模拟器进行发射率检索：挑战和建议

摘要。光谱发射率是地球表面的一个关键特性，到目前为止，在远红外 (FIR) 光谱区域中只有很少的测量值存在，尽管最近的工作表明它的 FIR 贡献对于全球气候的准确建模很重要。欧洲航天局的第 9 次地球探索者 FORUM（远红外输出辐射理解和监测）将提供 FIR 中地球大气顶 (TOA) 频谱的首次全球光谱分辨测量。在水汽含量低的晴朗天空条件下，这些测量将提供一个独特的机会来检索光谱解析的 FIR 表面发射率。为准备于 2026 年启动的 FORUM 任务，本研究通过调查全光谱最佳估计反演方法的发射率产品对不同物理和操作参数的敏感性，迈出了为 FORUM 开发业务发射率反演的第一步。用于敏感性测试的工具是 FORUM 任务的端到端模拟器。这些测试表明，对于 350-600 cm 范围内的干燥场景，大多数表面类型的光谱发射率都可以恢复。⁻¹区域的不确定性范围为 0.005 到 0.01。此外，还根据现场可降水的水汽含量对反演质量进行了量化，并研究了由发射率与地表温度的相关性引起的不确定性。基于这些调查，提出了两个主要建议：(1) 由于 FIR 中 TOA 对表面的敏感性程度取决于大气状态，发射率产品的光谱区域应使用所谓的信息量化器来确定，计算从检索不确定性与先验不确定性的比率。(2) 根据反演输入参数，发射率与表面温度的相关性允许反演发射率在真实发射率附近的小范围内。