In this paper, we explore the determination of a spectral emissivity profile that closely matches real data, intended for use as an initial guess and/or a priori information in a retrieval code. Our approach employs a Bayesian method that integrates the CAMEL (Combined ASTER MODIS Emissivity over Land) emissivity database with the MODIS/Terra+Aqua Yearly Land Cover Type database. The solution is derived as a convex combination of high-resolution Huang profiles using the Bayesian framework. We test our method on IASI (Infrared Atmospheric Sounding Interferometer) data and find that it outperforms the linear spline interpolation of the CAMEL data and the Huang emissivity database itself.

Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) was selected in 2019 as the ninth Earth Explorer mission by the European Space Agency. Its primary objective is to collect interferometric measurements in the far-infrared (FIR) spectral range, which accounts for 50% of Earth's outgoing longwave radiation emitted into space, and will be observed from space for the first time. Accurate measurements of the FIR at the top of the atmosphere are crucial for improving climate models. Current instruments are insufficient, necessitating the development of advanced computational techniques. FORUM will provide unprecedented insights into key atmospheric parameters, such as surface emissivity, water vapor, and ice cloud properties, through the use of a Fourier transform spectrometer. To ensure the quality of the mission's data, an end-to-end simulator was developed to simulate the measurement process and evaluate the effects of instrument characteristics and environmental factors. The core challenge of the mission is solving the retrieval problem, which involves estimating atmospheric properties from the radiance spectra observed by the satellite. This problem is ill-posed and regularization techniques are necessary to stabilize the solution. In this work, we present a data-driven approach to approximate the inverse mapping in the retrieval problem, aiming to achieve a solution that is both computationally efficient and accurate. In the first phase, we generate an initial approximation of the inverse mapping using only simulated FORUM data. In the second phase, we improve this approximation by introducing climatological data as a priori information and using a neural network to estimate the optimal regularization parameters during the retrieval process. While our approach does not match the precision of full-physics retrieval methods, its key advantage is the ability to deliver results almost instantaneously, making it highly suitable for real-time applications. Furthermore, the proposed method can provide more accurate a priori estimates for full-physics methods, thereby improving the overall accuracy of the retrieved atmospheric profiles.

In this contribution, we provide an overview of the EMM (Earth and Mars Research Network) research infrastructure, outlining its main components and the potential scientific products that can be derived through its use. The presentation delves into selected aspects in greater detail, particularly where they resonate with the multiple scientific and technological challenges associated with the FORUM mission.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) has been approved to be the ninth Earth Explorer mission of the European Space Agency and is scheduled for launch in 2027. The core FORUM instrument is a Fourier transform spectrometer, which will, for the first time, measure the upwelling spectral radiance in the far-infrared (FIR) and mid-infrared (MIR) portions of the Earth’s spectrum. These radiances will be processed up to level 2, to determine mainly the vertical profile of water vapor, surface spectral emissivity, and cloud parameters. In this paper, we assess the performance of the FORUM surface spectral emissivity product based on all-sky sensitivity study. In the FIR, we find that the retrieval error is mainly driven by the precipitable water vapor (PWV) in clear-sky conditions. In dry atmospheres, FIR emissivity can be retrieved with an error less than 0.01. In cloudy conditions, small errors can be achieved for optically thin clouds, especially for small values of the PWV. In the MIR, we observe that a large thermal contrast between the surface and the lowest atmospheric layers increases the sensitivity of the measurements to the surface emissivity in clear-sky conditions and an emissivity retrieval error less than 0.01 can usually be achieved. In cloudy conditions, small errors can be achieved for optically thin clouds, especially for large values of the surface temperature. Applying a coarser retrieval grid further reduces retrieval error, at the expense of an increased emissivity smoothing error.

The new σ-IASI/F2N radiative transfer model is an advancement of the σ-IASI model, introduced in 2002. It enables rapid simulations of Earth-emitted radiance and Jacobians under various sky conditions and geometries, covering the spectral range of 3-100 μm. Successfully utilized in δ-IASI, the advanced Optimal Estimation tool tailored for the IASI MetOp interferometer, its extension to the Far Infrared (FIR) holds significance for the ESA Earth Explorer FORUM mission, necessitating precise cloud radiative effect treatment, crucial in regions with dense clouds and temperature gradients. The model's update, incorporating the "linear-in-T" correction, addresses these challenges, complementing the "linear-in-tau" approach. Demonstrations highlight its effectiveness in simulating cloud complexities, with the integration of the "linear-in-T" and Tang correction for the computation of cloud radiative effects. The results presented will show that the updated σ-IASI/F2N can treat the overall complexity of clouds effectively and completely, at the same time minimizing biases.

Starting from 2019, the Italian Space Agency (ASI) is supporting dedicated projects for the development of new methods, tools and competences for the interpretation and the exploitation of the future measurements of the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment. FORUM will be the ninth Earth Explorer mission of the European Space Agency, scheduled for launch on a polar orbiting satellite in 2027. The core instrument of the mission will be a Fourier Transform Spectrometer with spectral range extending down to the Far-InfraRed (FIR), from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), thus covering the whole Earth's outgoing longwave radiation spectrum. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). The Middle-InfraRed (MIR) range (645 to 2760 cm-1) of the upwelling atmospheric spectrum measured by IASI-NG will effectively complement the FORUM measurement. All together, the two missions will provide matching spectral radiance measurements with unprecedented coverage, from 100 to 2760 cm-1. While the FIR part of the spectrum (100-667 cm-1) measured by FORUM is the most sensitive to the water vapour content in the UTLS and to ice cloud properties, the atmospheric windows in the MIR are measured by IASI-NG with a very high signal-to-noise ratio, thus supplying very precise information on the surface temperature and on the temperature profile, which are essential to constrain the retrieval of the other geophysical parameters. To get ready for the exploitation of the synergistic FORUM and IASI-NG measurements, in the frame of the mentioned ASI projects a Bayesian retrieval algorithm with the capability to perform the simultaneous inversion of two different synergistic spectral radiance measurements was developed. The tool is named FAst Retrieval Model (FARM) as it is based on a fast monochromatic and parametrized radiative transfer model (?-IASI) which is also being further extended and refined within the same projects. FARM includes the capability to retrieve simultaneously both atmospheric and cloud parameters. Furthermore, the code can handle both air/space- borne nadir measurements and ground- based zenith measurements. In this work, we introduce the functionalities of the developed algorithm and present the results of the self-consistency and verification tests. The preliminary results of the inversion of some existing real measurements are also discussed.

The simulations for the inverse problem of radiative transfer, even if built with a correct Bayesian approach, do not represent the full source of errors present in the experimental data. We point out two categories of errors (atmospheric model errors and non-Gaussian instrumental errors due to the optics and hardware, that are not considered by standard methods. Moreover, we show cases taken from FORUM simulated radiances using an End to End simulator, where se show how the instrument reacts to a non homogeneousneous filed of view.

MIPAS is a Fourier Transform spectrometer that measured the atmospheric limb emission spectra in the middle infrared on board the ENVISAT satellite. These measurements allowed the global monitoring of the three-dimensional (latitude, longitude and altitude) distribution of temperature and of the concentrations of many species, during both day and night, for 10 years, from July 2002 to April 2012. MIPAS measurements allowed to study the atmosphere from the upper troposphere to the stratosphere and above, up to the thermosphere. The interest in these measurements goes beyond the end of the mission, as they can be used in long time series of data to determine changes in atmospheric composition and in our planet's climate. Furthermore, if the Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) mission, one of four candidates for Earth Explorer 11, will be selected, MIPAS data will constitute a benchmark for these measurements. CAIRT exploits indeed the heritage of MIPAS on ENVISAT, but allows to measure the composition of the atmosphere with unprecedented three-dimensional resolution being the first imaging Fourier Transform spectrometer sounding the limb of the atmosphere from space. For the last reanalysis of the whole MIPAS mission, a significant effort was made by the MIPAS Quality Working Group, supported by ESA, to improve both L1 [1] and L2 processors, as well as spectroscopy and Level 2 Initial Guess profiles [2], with the objectives of obtaining L2 products with increased accuracy, better temporal stability, and a larger number of retrieved species. The main improvements of L1 processor were related to the radiometric calibration and pointing. With these new processors a MIPAS full mission reprocessing has been recently performed ([1] and [3]). The quality of this final operational data set has been assessed with comprehensive validation studies including comparisons to ground-based in-situ and balloon-borne measurements. The dataset containing the new version 8 of both L1 and L2 products and covering the entire MIPAS operational lifetime period (2002-2012) is available at ESA Earth Online web site.This paper will focus on the lessons learnt, on the quality of the reprocessed data, on the remaining problems, and on further improvements that could improve the quality of both MIPAS L1 and L2 datasets.[1] Kleinert et al. Kleinert, A., Birk, M., Perron, G., and Wagner, G.: Level 1b error budget for MIPAS on ENVISAT, Atmos. Meas. Tech., 11, 5657-5672,https://doi.org/10.5194/amt-11-5657-2018, 2018 [2] Raspollini, P., Arnone, E., Barbara, F., Bianchini, M., Carli, B., Ceccherini, S., Chipperfield, M. P., Dehn, A., Della Fera, S., Dinelli, B. M., Dudhia, A., Flaud, J.-M., Gai, M., Kiefer, M., López-Puertas, M., Moore, D. P., Piro, A., Remedios, J. J., Ridolfi, M., Sembhi, H., Sgheri, L., and Zoppetti, N.: Level 2 processor and auxiliary data for ESA Version 8 final full mission analysis of MIPAS measurements on ENVISAT, Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2021-235, in review, 2021. [3] Dinelli, B. M., Raspollini, P., Gai, M., Sgheri, L., Ridolfi, M., Ceccherini, S., Barbara, F., Zoppetti, N., Castelli, E., Papandrea, E., Pettinari, P., Dehn, A., Dudhia, A., Kiefer, M., Piro, A., Flaud, J.-M., Lopez-Puertas, M., Moore, D., Remedios, J., and Bianchini, M.: The ESA MIPAS/ENVISAT Level2-v8 dataset: 10 years of measurements retrieved with ORM v8.22, Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2021-215, accepted, 2021.

We report the regularization technique used in the inversion of the radiative transfer equation. The talk is intended for a general public, and does not require specialistic notions.

We report the results of the regularization of the surface emissivity for the FORUM instrument.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) is a Fourier Transform Spectrometer (FTS) that will fly as the 9th ESA's Earth Explorer mission. FORUM will sound the atmosphere in the 100-1600 cm-1 region, covering the Far Infrared (FIR) and part of the Middle Infrared (MIR), accounting for more than 95% of the outgoing longwave flux lost by our planet. We review the constrains for the emissivity retrieval.

High quality long-term data sets of altitude-resolved measurements of the atmospheric composition areimportant because they can be used both to study the evolu-tion of the atmosphere and as a benchmark for future mis-sions. For the final ESA reprocessing of MIPAS (MichelsonInterferometer for Passive Atmospheric Sounding) on ENVISAT (ENViromental SATellite) data, numerous improve-ments were implemented in the Level 2 (L2) processor Op-timised Retrieval Model (ORM) version 8.22 (V8) and itsauxiliary data. The implemented changes involve all aspects of the processing chain, from the modelling of the measure-ments with the handling of the horizontal inhomogeneitiesalong the line of sight to the use of the optimal estimationtechnique to retrieve the minor species, from a more sensitive approach to detecting the spectra affected by cloudsto a refined method for identifying low quality products. Improvements in the modelling of the measurements werealso obtained with an update of the used spectroscopic dataand of the databases providing the a priori knowledge ofthe atmosphere. The HITRAN_mipas_pf4.45 spectroscopic database was finalised with new spectroscopic data verifiedwith MIPAS measurements themselves, while recently measured cross-sections were used for the heavy molecules. TheLevel 2 Initial Guess (IG2) data set, containing the clima-tology used by the MIPAS L2 processor to generate the ini-tial guess and interfering species profiles when the retrieved profiles from previous scans are not available, was improved taking into account the diurnal variation of the profiles defined using climatologies from both measurements and models. Horizontal gradients were generated using the ECMWFERA-Interim data closest in time and space to the MIPASdata. Further improvements in the L2 V8 products derivedfrom the use of the L1b V8 products, which were upgraded to reduce the instrumental temporal drift and to handle theabrupt changes in the calibration gain. The improvements in-troduced into the ORM V8 L2 processor and its upgraded auxiliary data, together with the use of the L1b V8 products, lead to the generation of the MIPAS L2 V8 products, which are characterised by an increased accuracy, better temporal stability and a greater number of retrieved species.

Spectral emissivity is a key property of the Earth's surface, of which only very few measurements exist so farin the far-infrared (FIR) spectral region, even though recent work has shown that the FIR is important for accurate mod-elling of the global climate. The European Space Agency's9th Earth Explorer, FORUM (Far-infrared Outgoing Radi-ation Understanding and Monitoring) will provide the firstglobal spectrally resolved measurements of the Earth's top-of-the-atmosphere (TOA) spectrum in the FIR. In clear-skyconditions 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 2027, this study takes the first steps towards the development of an opera-tional emissivity retrieval for FORUM by investigating the sensitivity of the emissivity product of a full spectrum optimal estimation retrieval method to different physical andoperational parameters. The tool used for the sensitivity testsis the FORUM mission's end-to-end simulator. These tests show that the spectral emissivity of most surface types canbe retrieved for dry scenes in the 350-600 cm-1 region, with an absolute uncertainty ranging from 0.005 to 0.01. In addition, the quality of the retrieval is quantified with respectto the precipitable water vapour content of the scene, and the uncertainty caused by the correlation of emissivity withsurface temperature is investigated. Based on these investigations, a road map is recommended for the development of the operational emissivity product.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) will fly as the ninth ESA's Earth Explorer mission, and an end-to-end simulator (E2ES) has been developed as a support tool for the mission selection process and the subsequent development phases. The current status of the FORUM E2ES project is presented together with the characterization of the capabilities of a full physics retrieval code applied to FORUM data. We show how the instrument characteristics and the observed scene conditions impact on the spectrum measured by the instrument, accounting for the main sources of error related to the entire acquisition process, and the consequences on the retrieval algorithm. Both homogeneous and heterogeneous case studies are simulated in clear and cloudy conditions, validating the E2ES against appropriate well-established correlative codes. The performed tests show that the performance of the retrieval algorithm is compliant with the project requirements both in clear and cloudy conditions. The far-infrared (FIR) part of the FORUM spectrum is shown to be sensitive to surface emissivity, in dry atmospheric conditions, and to cirrus clouds, resulting in improved performance of the retrieval algorithm in these conditions. The retrieval errors increase with increasing the scene heterogeneity, both in terms of surface characteristics and in terms of fractional cloud cover of the scene.

Questo documento riassume l'attività svolta nei vari WP, le azioni completate e lo stato di del progetto progetto per il periodo di attività dal 9 giugno 2021 (RA3) al 19 aprile 2022 (RA4)

o The first draft version of the Interface definition report has been delivered at PM2. The draft pointed out the various problems and solutions in defining the netCDF interface. At PM2 it was decided to concentrate the efforts on defining the output file interfaces, since these are the main files that will be used to analyze the result, both in the FORUM Scienza community, and in the general remote sensing community. o The second draft of the document was delivered at PM3. It contained the specification of the output interface. o This final version is delivered at PM4 and contains the revised specification for the output files. o Example fortran 95 routines that read and write a compliant output file are also delivered. These also include an extensive documentation on how to interface a proprietary code with the supplied routines.

We describe studies undertaken in support of the Far-infrared Outgoing Radiation Understanding and Monitoring mission, European Space Agency's ninth Earth Explorer, designed to investigate whether airborne observations of far-infrared radiances can provide beneficial information on mid and upper tropospheric water vapor concentrations. Initially we perform a joint temperature and water vapor retrieval and show that the water vapor retrieval exploiting far-infrared measurements from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) shows improvement over the a-priori Unified Model global forecast when compared to in situ dropsonde measurements. For this case the improvement is particularly noticeable in the mid-upper troposphere. Equivalent retrievals using mid-infrared radiances measured by the Airborne Research Interferometer Evaluation System (ARIES) show much reduced performance, with the degrees of freedom for signal (DFS), reduced by a factor of almost 2. Further sensitivity studies show that this advantage is decreased, but still present when the spectral resolution of the TAFTS measurements is reduced to match that of ARIES. The beneficial role of the far infrared for this case is further confirmed by performing water vapor only retrievals using ARIES and TAFTS individually, and then in combination. We find that the combined retrieval has a DFS value of 6.7 for water vapor, marginally larger than that obtained for the TAFTS retrieval and almost twice as large as that obtained for ARIES. These results provide observational support of theoretical studies highlighting the potential improvement that far-infrared observations could bring for the retrieval of tropospheric water vapor.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) will flight as the 9th ESA's Earth Explorer mission, and an End-to-End Simulator (E2ES) has been developed as a support tool for the mission selection process and the subsequent development phases. The current status of the FORUM E2ES project is presented, together with the characterization of the capabilities of a full physics retrieval code applied to FORUM data. We show how the instrument characteristics and5the observed scene conditions impact on the spectrum measured by the instrument, accounting for the main sources of error related to the entire acquisition process, and the consequences on the retrieval algorithm. Both homogeneous and heterogeneous case studies are simulated in clear and cloudy conditions, validating the E2ES against two independent codes: KLIMA (clear sky) and SACR (cloudy sky). The performed tests show that the performance of the retrieval algorithm is compliant with the project requirements both in clear and cloudy conditions. The far infrared (FIR) part of the FORUM spectrum is shown to be10sensitive to surface emissivity, in dry atmospheric conditions, and to cirrus clouds, resulting in improved performance of the retrieval algorithm in these conditions. The retrieval errors increase with increasing the scene heterogeneity, both in terms of surface characteristics and in terms of fractional cloud cover of the scene.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measured the middle-infrared limb emission spectrum of the atmosphere from 2002 to 2012 on board ENVISAT, a polar-orbiting satellite. Recently, the European Space Agency (ESA) completed the final reprocessing of MIPAS measurements, using version 8 of the level 1 and level 2 processors, which include more accurate models, processing strategies, and auxiliary data. The list of retrieved gases has been extended, and it now includes a number of new species with weak emission features in the MIPAS spectral range. The new retrieved trace species include carbonyl chloride (COCl2), also called phosgene. Due to its toxicity, its use has been reduced over the years; however, it is still used by chemical industries for several applications. Besides its direct injection in the troposphere, stratospheric phosgene is mainly produced from the photolysis of CCl4, a molecule present in the atmosphere because of human activity. Since phosgene has a long stratospheric lifetime, it must be carefully monitored as it is involved in the ozone destruction cycles, especially over the winter polar regions.In this paper we exploit the ESA MIPAS version 8 data in order to discuss the phosgene distribution, variability, and trends in the middle and lower stratosphere and in the upper troposphere. The zonal averages show that phosgene volume mixing ratio is larger in the stratosphere, with a peak of 40 pptv (parts per trillion by volume) between 50 and 30 hPa at equatorial latitudes, while at middle and polar latitudes it varies from 10 to 25 pptv. A moderate seasonal variability is observed in polar regions, mostly between 80 and 50 hPa. The comparison of MIPAS-ENVISAT COCl2 v8 profiles with the ones retrieved from MIPAS balloon and ACE-FTS (Atmospheric Chemistry Experiment - Fourier Transform Spectrometer) measurements highlights a negative bias of about 2 pptv, mainly in polar and mid-latitude regions. Part of this bias is attributed to the fact that the ESA level 2 v8 processor uses an updated spectroscopic database. For the trend computation, a fixed pressure grid is used to interpolate the phosgene profiles, and, for each pressure level, VMR (volume mixing ratio) monthly averages are computed in pre-defined 10? wide latitude bins. Then, for each latitudinal bin and pressure level, a regression model has been fitted to the resulting time series in order to derive the atmospheric trends. We find that the phosgene trends are different in the two hemispheres. The analysis shows that the stratosphere of the Northern Hemisphere is characterized by a negative trend of about -7 pptv per decade, while in the Southern Hemisphere phosgene mixing ratios increase with a rate of the order of +4 pptv per decade. This behavior resembles the stratospheric trend of CCl4, which is the main stratospheric source of COCl2. In the upper troposphere a positive trend is found in both hemispheres.

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.