In this talk we describe the scientific motivation for the FORUM mission. We will demonstrate why the mission is needed, focusing on the exploratory nature of the planned observations and the insights we expect them to provide. In particular, we will highlight why observing the far-infrared radiation spectrum is critical to understand our evolving climate, demonstrating how FORUM’s spectrally resolved measurements can be expected to provide new insights into key climate drivers and their radiative feedback.

As part of the talk we will describe both the anticipated level 1c and level 2 products, including a description of the proposed level 2 processer.

ESA is currently developing the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission that has the important ambition to record for the first time the emission spectrum of the Earth’s stratosphere and troposphere in the spectral range of 100 to 1600 cm-1 (between 6.25μm to 100 μm). The large spectral range imposes challenges not just on the scientific community, but also on the engineering teams working on the spacecraft and payload design and requiring the development of innovative infrastructures. The payload will be constituted by two instruments, the Sounding Instrument (FSI), a Fourier Transform Spectrometer (FTS). The second instrument will be a thermal infrared imager called FORUM Embedded Imager (FEI) co-located within the same footprint of the FSI. The FTS instrument will be scanning the Earth atmosphere in a step-and-stare acquisition mode sounding a 15 km diameter ground sampling with a ground sampling distance of 100 km, and providing a spectral resolution better than 0.5cm-1, while the FEI with a ground sampling of 36x36 km2. The presentation will show how the mission requirements will be met by the FORUM payload including giving an overview of the main technological pre-development activities and their technical challenges.

The measurements of the FIR (Far InfraRed) portion of the spectral radiance emitted by the Earth’s atmosphere, the main goal of the FORUM mission, were primarily exploited by a few ground-based and air-borne prototypes. In this presentation, a review of the main results obtained in recent years from the field deployment of two Fourier transform spectroradiometers (FTS) developed at CNR, REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) and FIRMOS (Far-Infrared Radiation Mobile Observation System) is presented.

REFIR-PAD is the first FTS where technology developments allowed uncooled high-accuracy operations over a wideband spectral region. The instrument performs spectral radiance measurements covering the 100 to 1500 cm^-1 range with a resolution of 0.4 cm^-1, at different viewing directions: nadir, zenith, limb and deep space view at +30° elevation angle in the balloon-application. The capability to provide a full spectral characterisation of the atmospheric OLR (Outgoing Long-wave Radiation) emission was assessed during a stratospheric balloon flight in 2005 from Teresina (Brazil). Subsequently, on three different ground-based campaigns, the capability for measuring DLR (Downward Long-wave Radiation) was also established.

The OLR results from the 2005 flight made possible the retrieval of the atmospheric state with high precision, leading to a more accurate calculation of the integrated outgoing radiation flux. It was thereby demonstrated that spectral information can be used to infer the angular distribution of radiance. We have observed that the error of our flux measurement is about 1 W/m^2 and it is mainly due to the radiometric calibration uncertainty, rather than the random detector noise. This result proves the feasibility of climatological studies with instruments employing noisy uncooled detectors (Palchetti et al., 2008). The measured spectra have also been used to test the performance of the various water vapour spectroscopic databases (Di Roma et al. 2021).

The results from the ground-based deployment of REFIR-PAD show the capability of these measurements to improve the radiative transfer codes used to simulate the spectral radiances in the FIR. The ECOWAR (Earth COoling by Water vapor Radiation) field campaign in the Italian Alps (Testa Grigia, 3500 m AMSL) and RHUBC‐II (Radiative Heating in Underexplored Bands Campaign) from Chile (Cerro Toco, 5380 m AMSL) allowed us to revise and improve the MT_CKD continuum model mostly in the range 260–590 cm^-1 (Masiello et al., 2012, Mlawer et al., 2019). The permanent installation in 2011 of REFIR-PAD over the Antarctic plateau at Concordia Station (Dome-C, 3220 m AMSL), where it almost continuously measures the DLR, allowed us to extend the validation and verification of state-of-art water vapour continuum absorption models down to 180 cm^-1 (Liuzzi et al., 2014).

REFIR-PAD measurements were also used to test machine learning algorithms for cloud identification and classification (Cossich et al., 2021) and to derive optical and microphysical properties of cirrus clouds (Maestri et al., 2019). The FIR spectral region allows to improve the quality of the retrieval of cirrus properties, which critically influence the Earth radiation budget and climate sensitivity (Maestri et al., 2014, Palchetti et al., 2016).

After REFIR-PAD was permanently installed in Antarctica, in 2018 a new prototype FIRMOS was developed to support the preparation of the feasibility phase of the FORUM mission both with laboratory measurements and field applications. A ground-based campaign was performed at the Zugspitze summit, Germany (2962 m AMSL) that offers excellent facilities to provide an independent and complete characterisation of the observed atmospheric state. The instrument was operated for two months in November-December 2018 and January-February 2019 in favourable weather conditions. FIRMOS covers the 100-1000 cm^-1 range with a resolution of 0.36 cm^-1 and 36 s acquisition time. The calibrated measurements were validated using E-AERI (Extended-range Atmospheric Emitted Radiance Interferometer), an operational instrument installed permanently at the summit covering the 400 to 3000 cm^-1 spectral range with 0.6 cm^-1 of resolution (Sussmann et al., 2016).

FIRMOS measurements were used to retrieve the atmospheric parameters with the KLIMA code (Del Bianco et al. 2014) in clear sky conditions and with the SACR code (Di Natale et al. 2020) for cloudy scenes. The retrieved vertical profiles and cloud parameters show a good agreement respectively with the standard products of the closest radio soundings and with the backscattering Lidar, operated from the nearby research station of the Schneefernerhaus, about 600 m away from the summit. During the campaign, FIRMOS was also used to measure the surface spectral emission of snow that is not well-characterised in the FIR range. Measurements were performed by adapting FIRMOS to observe in a slant direction close to nadir providing one of the first measurement capable to sound the FIR snow properties down to 180 cm^-1.

The described results show the capability of spectroradiometers like REFIR-PAD and FIRMOS to characterise the radiative properties of water vapour, cirrus clouds and snow emissivity in the under-explored FIR spectral range. They also demonstrate that such instruments can be used to provide valuable datasets to the atmospheric modelling community as well as a methodology to improve the scientific readiness level of the FORUM mission.

References

- Cossich Marcial De Farias W.; Maestri T.; Magurno D.; Martinazzo M.; Di Natale G.; Palchetti L.; Bianchini G.; Del Guasta M., Ice and mixed-phase cloud statistics on the Antarctic Plateau, «ATMOSPHERIC CHEMISTRY AND PHYSICS», 2021, 21, pp. 13811 - 13833

- Del Bianco, S., B. Carli, M. Gai, L.M. Laurenza, U. Cortesi (2014), XCO2 retrieved from IASI using KLIMA algorithm, Annals of Geophysics, 56, doi:10.4401/ag-6331.

- Di Natale G, Palchetti L, Bianchini G and Ridolfi M (2020), "The two-stream δ-Eddington approximation to simulate the far infrared Earth spectrum for the simultaneous atmospheric and cloud retrieval", Journal of Quantitative Spectroscopy and Radiative Transfer. Vol. 246, pp. 106927.

- Di Roma A., Dinelli B.M., Castelli E., Palchetti L., Bianchini G., Belotti C., Warwick L., Murray J., Brindley H. (2021): Analysis of the Consistency Between FIR Spectroscopic Data and Airborne Measurements: Support to FORUM-EE9 Mission, poster at the ESA conference ATMOS-2021.

- Liuzzi G., Masiello G., Serio C., Palchetti L., Bianchini B. (2014), "Validation of H2O continuum absorption models in the wave number range 180–600 cm−1 with atmospheric emitted spectral radiance measured at the Antarctica Dome-C site," Opt. Express 22, 16784-16801.

- Maestri T, Rizzi R, Tosi E, Veglio P, Palchetti L, Bianchini G, Di Girolamo P, Masiello G, Serio C and Summa D (2014), "Analysis of cirrus cloud spectral signatures in the far infrared", Journal of Quantitative Spectroscopy and Radiative Transfer. Vol. 141, pp. 49-64.

- Maestri, T., Arosio, C., Rizzi, R., Palchetti, L., Bianchini, G., Del Guasta, M. (2019). Antarctic ice cloud identification and properties using downwelling spectral radiance from 100 to 1,400 cm−1, Journal of Geophysical Research: Atmospheres, 124, 4761–4781. https://doi.org/10.1029/2018JD029205

- Masiello G, Serio C, Esposito F and Palchetti L (2012), "Validation of line and continuum spectroscopic parameters with measurements of atmospheric emitted spectral radiance from far to mid infrared wave number range", Journal of Quantitative Spectroscopy and Radiative Transfer. Vol. 113(11), pp. 1286-1299.

- Mlawer, E. J., Turner, D. D., Paine, S. N., Palchetti, L., Bianchini, G., Payne, V. H., et al. (2019). Analysis of water vapor absorption in the far‐infrared and submillimeter regions using surface radiometric measurements from extremely dry locations. Journal of Geophysical Research: Atmospheres, 124, 8134–8160. https://doi.org/10.1029/2018JD029508

- Palchetti, L., Bianchini, G., Carli, B., Cortesi, U., and Del Bianco, S. (2008): Measurement of the water vapour vertical profile and of the Earth's outgoing far infrared flux, Atmos. Chem. Phys., 8, 2885–2894, https://doi.org/10.5194/acp-8-2885-2008.

- Palchetti, L., G. Di Natale, G. Bianchini (2016), Remote sensing of cirrus cloud microphysical properties using spectral measurements over the full range of their thermal emission, J. Geophys. Res. Atmos., 121, 10, 804-819.

- Sussmann, R., A. Reichert, M. Rettinger, (2016), The Zugspitze radiative closure experiment for quantifying water vapor absorption over the terrestrial and solar infrared – Part 1: Setup, uncertainty analysis, and assessment of far-infrared water vapor continuum, Atmos. Chem. Phys., 16, 11649-11669.

In this talk we describe three scientific studies using the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) undertaken in support of the FORUM mission. The observations exploited for these studies were taken during two airborne field campaigns of the UK Facility for Airborne Atmospheric Measurements (FAAM) Bae-146 aircraft. The observational suite included not only the TAFTS far-infrared measurements (nominally covering 80-600 cm-1 at 0.12 cm-1 resolution), but also co-incident spectrally resolved mid-infrared observations from the UK Met Office Airborne Research Interferometer Evaluation System (ARIES) as well as on-board in-situ sensors and atmospheric profiling capability.

Although the radiometric instrumentation is not an exact match to FORUM in terms of spectral resolution, radiometric noise, and calibration uncertainty, it can be used to illustrate the potential value of far-infrared observations. For example, we will demonstrate how the observations from TAFTS and ARIES have been used synergistically to derive the first estimates of high latitude surface emissivity from an airborne platform. Similarly, we will show how the TAFTS measurements result in an improved estimate of mid-upper humidity compared to the use of ARIES alone for a clear-sky test case. Finally, we highlight the inability of current state-of-the-art cirrus bulk optical property models to simultaneously match above cirrus radiance observations across the mid and far infrared for two different case studies. These observational studies indicate how FORUM can benefit our knowledge of several key variables that strongly influence radiative energy transfers in the Earth system.

If time permits we will also briefly discuss plans for the development of a successor to TAFTS, the Universal InfraRed Airborne Spectrometer (UNIRAS). Designed to cover both the far and mid-infrared at a spectral resolution commensurate with FORUM, this will provide a flexible airborne observational capability in support of FORUM and other planned ESA missions.

Better knowledge on the water vapour feedback is important for improved predictions of climate change. Here, we investigate variations in OLR driven by inter-annual trace gas variability. In particular, we discuss how well such variations allow for estimating feedback strength. Previous studies using this approach are in good agreement with present calculations of the water vapor feedback strength from climate simulations. We present water vapour simulations of CLaMS driven by ECMWF reanalysis data combined with radiative transfer calculations of JURASSIC providing global distributions of OLR. Calculated are changes in OLR by water vapor perturbations, mainly induced by ENSO (El Nino Southern Oscillation). During ENSO the main convective region of the Walker-cell over the Pacific is shifting within several months with a corresponding large signal in water vapor. Our work is of special interest for the FORUM mission, which aim to constrain the water vapor feedback despite its relatively short lifetime.

The climate feedback parameter λ quantifies, how Earth’s radiation balance responds to warming and strongly impacts Earth’s climate sensitivity. Usually, the discussion of radiative feedbacks in the climate literature is limited to the spectrally integrated λ. However, this approach misses important details. Analysing the spectrally resolved feedback parameter λ_ν provides insights into feedback processes
that can compensate in the integrated λ. While some studies have investigated λ_ν based on models, no study the authors are aware of has attempted to derive λ_ν from observations.
To close this gap, this study derives λ_ν from satellite observations by the hyperspectral Infrared Atmospheric Sounding Interferometer (IASI). As the available time series is too short to employ traditional approaches, we use a method based on interannual variability, which has been shown to produce robust estimates of the integrated λ. We use interannual variations in the spectrally resolved outgoing longwave radiation (OLR_ν) and the near-surface air temperature to infer the longwave λ_ν for both clear-sky and all-sky conditions.
Due to the lack of satellite observations with high spectral resolution continuously covering the farinfrared, this approach is currently limited to the mid-infrared spectral range covered by IASI (645–2760 cm−1). However, the launch of the Far-infrared-Outgoing-Radiation Understanding and Monitoring (FORUM) mission will make it possible to expand this analysis to also infer λ_ν in the far-infrared. To estimate how FORUM’s contribution to estimating λ_ν in the far-infrared might look like, we additionally perform OLR_ν simulations of the combined spectral ranges of IASI and FORUM (100–2760 cm−1). Using state-of-the-art climate models for this also allows to investigate how well those models can reproduce the spectral signature present in the satellite observations.
The results demonstrate the feasibility of inferring λ_ν from satellite observations. The inferred λ_ν from satellite observations are on average well reproduced by the climate models, there is however substantial inter-model spread. The largest systematic discrepancies between observations and models occur in the mid-infrared atmospheric window, where the observed λ_ν is more negative compared to
the reference simulations.