Retrievals of water vapour and temperature exploiting the far-infrared: application to aircraft observations in preparation for the FORUM mission

Atmos. Meas. Tech., 18, 717–735, 2025 https://doi.org/10.5194/amt-18-717-2025 © Author(s) 2025. This work is distributed under the Creative Commons Attribution 4.0 License. Retrievals of water vapour and temperature exploiting the far-infrared: application to aircraft observations in preparation for the FORUM mission Sanjeevani Panditharatne1,2,3 , Helen Brindley1,3 , Caroline Cox2 , Richard Siddans2,4 , Jonathan Murray1,3 , Laura Warwick5 , and Stuart Fox6 1 Department of Physics, Imperial College London, London, UK Space, Harwell Oxford, Chilton, UK 3 NERC National Centre for Earth Observation, Imperial College London, London, UK 4 NERC National Centre for Earth Observation, RAL Space, Harwell Oxford, Chilton, UK 5 ESA-ESTEC, Noordwijk, the Netherlands 6 Met Office, Exeter, UK 2 RAL Correspondence: Sanjeevani Panditharatne () Received: 31 July 2024 – Discussion started: 7 August 2024 Revised: 29 October 2024 – Accepted: 9 December 2024 – Published: 10 February 2025 Abstract. We present the extension of the Rutherford Appleton Laboratory (RAL) Infrared Microwave Sounding (IMS) optimal estimation retrieval scheme to include the use of far-infrared channels in preparation for the upcoming Farinfrared Outgoing Radiation Understanding and Monitoring (FORUM) mission. The IMS code has been previously applied to mid-infrared spectral radiances measured by the Infrared Atmospheric Sounding Instrument (IASI) to retrieve temperature and water vapour. Given this, the evolution and evaluation of the extended scheme is performed in two steps. First, clear-sky retrievals of temperature and water vapour are performed on IASI and FORUM simulations. Comparable retrieval biases are observed for retrievals of temperature and water vapour; however, there is an increase of ∼ 1 degree of freedom for water vapour and temperature for the FORUM configuration. Secondly, radiances observed from an aircraft flight in the upper troposphere are modified to match the FORUM spectral characteristics. Retrievals from these radiances using the modified code show a strong agreement with contemporaneous in situ measurements of the atmospheric state, reducing the root-mean-square error (RMSE) by 18 % for water vapour from the a priori, giving confidence in its performance. The extended IMS scheme is now available for use on FORUM observations and can be easily adapted to other far- and mid-infrared instrument configurations. 1 Introduction Water vapour composes approximately 3 % of the Earth’s atmosphere and is the most dominant greenhouse gas with absorption bands in the microwave, infrared, and visible spectral regions (Harries, 1996; Andrews, 2000). Capturing its spatial and temporal distribution is critical in quantifying the Earth’s greenhouse effect, characterising atmospheric circulation, and approximating the strength of water vapour’s radiative effect and feedback, which has the potential to exacerbate anthropogenic climate change (Dessler et al., 2008; Chung et al., 2014). Water vapour in the upper troposphere strongly regulates this water vapour feedback (Chung et al., 2014), with current increases in tropospheric moisture consistent with an amplifying water vapour feedback (IPCC, 2023). However, discrepancies in long-term trends exist between observations, climate models, and reanalysis datasets (Schröder et al., 2019; Santer et al., 2021; Allan et al., 2022). Considerable work has been performed to improve assessments of water vapour distributions through the use of in situ measurements such as radiosondes and aircraft-based sensors (Sun et al., 2021). However, best-case uncertainties from the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) radiosondes are estimated to still be of the order 5 %, reaching 15 % near the tropopause (Dirksen et al., Published by Copernicus Publications on behalf of the European Geosciences Union. 718 S. Panditharatne et al.: Retrievals of water vapour and temperature exploiting the far-infrared 2014), with limited homogeneity in the radiosondes’ distribution, regularity, and vertical sampling (Ferreira et al., 2019). Satellite observations across the electromagnetic spectrum have also been used to characterise the atmosphere with increased spatial coverage. However, satellite retrievals can lack sufficient vertical resolution (Chung et al., 2014), demonstrate an inherent bias (Santer et al., 2021), or have a reduced sensitivity to upper tropospheric water vapour (Kursinski and Gebhardt, 2014), causing inconsistencies between retrieved upper tropospheric humidities (Shi et al., 2022). While hyperspectral sounders, such as the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI), have improved the vertical resolution of water vapour and temperature (Chahine et al., 2006; Hilton et al., 2012), radiances in the mid-infrared region (667–2000 cm−1 ) have a limited sensitivity to water vapour in the mid- to upper troposphere, and upper tropospheric biases remain present (Fetzer et al., 2008; Trent et al., 2019). In the global mean, the far-infrared region (100– 667 cm−1 ) accounts for approximately 55 % of the outgoing longwave radiation (OLR), and the absorption of water vapour dominates this region, with its pure rotational band extending from 6 to 667 cm−1 (Brindley and Harries, 1998). As a consequence, radiances in this region are significantly more sensitive to middle and upper tropospheric water vapour than in the mid-infrared region (Sinha and Harries, 1995; Brindley and Harries, 1998). Due to technical limitations, there are historically no observations of spectrally resolved far-infrared radiances at the top of the atmosphere (TOA). However, this is changing due to space missions such as NASA’s Polar Radiant Energy in the Far-InfraRed Experiment and ESA’s Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, with the former having launched in summer 2024 and the latter scheduled for launch in 2027 (L’Ecuyer et al., 2021; Palchetti et al., 2020). This study is in support of the FORUM mission that aims to measure the Earth’s spectrally resolved OLR using the FORUM Sounding Instrument (FSI), which will have a spectral range from 100 to 1600 cm−1 with a spectral resolution greater than 0.5 cm−1 and a target radiometric accuracy of 0.1 K at 3σ . The FORUM satellite will fly in a loose formation with the EUMETSAT MetOp-SG-1A satellite, complementing mid-infrared observations of the OLR taken by the Infrared Atmospheric Sounding Instrument New Generation (IASI-NG); when combined, these observations will create a unique dataset of the Earth’s entire OLR spectrum (Palchetti et al., 2020). Given the sensitivities and spectral features across the farinfrared for water vapour, there is a significant potential for improved retrievals from the upcoming TOA observations in the far-infrared (Harries et al., 2008; Ridolfi et al., 2020). Theoretical studies by Merrelli and (...truncated)