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The long-term comparison between simulated and observed spectrally resolved outgoing longwave radiation (OLR) can represent a stringent test for the direct verification and improvement of general circulation models (GCMs), which are regularly tuned by adjusting parameters related to subgrid processes not explicitly represented in the model to constrain the integrated OLR energy fluxes to observed values. However, a good agreement between simulated and observed integrated OLR fluxes may be obtained from the cancellation of opposite-in-sign systematic errors localized in specific spectral ranges. Since the mid-2000s, stable hyperspectral observations of the mid-infrared region (667 to 2750 cm−1) of the Earth emission spectrum have been provided by different sensors (e.g. AIRS, IASI and CrIS). Furthermore, the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission, selected to be the ninth ESA Earth Explorer, will measure, starting from 2027, the terrestrial radiation emitted to space at the top of the atmosphere (TOA) from 100 to 1600 cm−1, filling the observational gap in the far-infrared (FIR) region, from 100 to 667 cm−1. In this work, in anticipation of FORUM measurements, we compare Infrared Atmospheric Sounding Interferometer (IASI) Metop-A observations to radiances simulated on the basis of the atmospheric fields predicted by the EC-Earth Global Climate Model (version 3.3.3) in clear-sky conditions. To simulate spectra based on the atmospheric and surface state provided by the climate model, the radiative transfer model σ-IASI has been integrated in the Cloud Feedback Model Intercomparison Project (COSP) package. Therefore, online simulations, provided by the EC-Earth model equipped with the new COSP–σ-IASI module, have been performed in clear-sky conditions with prescribed sea surface temperature and sea ice concentration, every 6 h, over a time frame consistent with the availability of IASI data. Systematic comparisons between observed and simulated brightness temperature (BT) have been performed in 10 cm−1 spectral intervals, on a global scale over the ocean, with a specific focus on the latitudinal belt between 30∘ S and 30∘ N. The analysis has shown a warm BT bias of about 3.5 K in the core of the CO2 absorption band and a cold BT bias of approximately 1 K in the wing of the CO2 band, due to a positive temperature bias in the stratosphere and a negative temperature bias in the middle troposphere of the climate model, respectively. Finally, considering a warm BT bias in the rotational–vibrational water vapour band, we have highlighted a dry bias of the water vapour concentration in the upper troposphere of the model.

The long-term comparison between simulated and observed spectrally resolved outgoing longwave radiation (OLR) can represent a stringent test for the direct verification and improvement of general circulation models (GCMs), which are regularly tuned by adjusting parameters related to subgrid processes not explicitly represented in the model to constrain the integrated OLR energy fluxes to observed values. However, a good agreement between simulated and observed integrated OLR fluxes may be obtained from the cancellation of opposite-in-sign systematic errors localized in specific spectral ranges. Since the mid-2000s, stable hyperspectral observations of the mid-infrared region (667 to 2750 cm−1) of the Earth emission spectrum have been provided by different sensors (e.g. AIRS, IASI and CrIS). Furthermore, the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission, selected to be the ninth ESA Earth Explorer, will measure, starting from 2027, the terrestrial radiation emitted to space at the top of the atmosphere (TOA) from 100 to 1600 cm−1, filling the observational gap in the far-infrared (FIR) region, from 100 to 667 cm−1. In this work, in anticipation of FORUM measurements, we compare Infrared Atmospheric Sounding Interferometer (IASI) Metop-A observations to radiances simulated on the basis of the atmospheric fields predicted by the EC-Earth Global Climate Model (version 3.3.3) in clear-sky conditions. To simulate spectra based on the atmospheric and surface state provided by the climate model, the radiative transfer model σ-IASI has been integrated in the Cloud Feedback Model Intercomparison Project (COSP) package. Therefore, online simulations, provided by the EC-Earth model equipped with the new COSP–σ-IASI module, have been performed in clear-sky conditions with prescribed sea surface temperature and sea ice concentration, every 6 h, over a time frame consistent with the availability of IASI data. Systematic comparisons between observed and simulated brightness temperature (BT) have been performed in 10 cm−1 spectral intervals, on a global scale over the ocean, with a specific focus on the latitudinal belt between 30∘ S and 30∘ N. The analysis has shown a warm BT bias of about 3.5 K in the core of the CO2 absorption band and a cold BT bias of approximately 1 K in the wing of the CO2 band, due to a positive temperature bias in the stratosphere and a negative temperature bias in the middle troposphere of the climate model, respectively. Finally, considering a warm BT bias in the rotational–vibrational water vapour band, we have highlighted a dry bias of the water vapour concentration in the upper troposphere of the model.

In the frame of Earth observation remote-sensing data analysis, synergistic retrieval (SR) and complete data fusion (CDF) are techniques used to exploit the complementarity of the information carried by different measurements sounding the same air mass and/or ground pixel. While more difficult to implement due to the required simultaneous access to measurements originating from different instruments, the SR method is sometimes preferred over the CDF method as the latter relies on a linear approximation of the retrieved states as functions of the true atmospheric and/or surface state. In this work, we study the performance of the SR and CDF techniques when applied to simulated measurements of the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) and the Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG) missions that will be operational in a few years, from two polar-orbiting satellites. The study is based on synthetic measurements generated for the two missions in clear-sky atmospheres. The target parameters of the inversion are the vertical profiles of temperature, water vapor and ozone mixing ratios, surface temperature, and spectral emissivity. We find that for exact matching of the measurements, the results of the SR and CDF techniques differ by less than 1/10 of their errors estimated through the propagation of measurement noise. For measurements with a realistic mismatch in space and time, the two methods provide more different results. Still in this case, however, the differences between the results are within the error bars due to measurement noise. We conclude that, when applied to FORUM and IASI-NG missions, the two methods are equivalent from an accuracy point of view.

In the frame of Earth observation remote sensing data analysis, Synergistic Retrieval (SR) and Complete Data Fusion (CDF) are techniques used to exploit the complementarity of the information carried by different measurements sounding the same air mass and / or ground pixel. While more difficult to implement due to the required simultaneous access to measurements originating from different instruments / missions, the SR method is sometimes preferred over the CDF method as the latter relies on a linear approximation of the retrieved states as functions of the true atmospheric and / or surface state. In this work, we study the performance of the SR and CDF techniques when applied to simulated measurements of the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) and the IASI-NG (Infrared Atmospheric Sounding Interferometer - New Generation) missions that will be operational in a few years, from two polar orbiting satellites. The study is based on synthetic measurements generated for the two missions, in clear-sky Antarctic atmospheres. The target parameters of the inversion are the vertical profiles of temperature, water vapour and ozone mixing ratios, surface temperature and spectral emissivity. We find that for exact matching of the measurements, the results of the SR and CDF techniques differ by less than 1/10 of their errors estimated trough the propagation of measurement noise. For measurements with a realistic mismatch in space and time, the two methods provide more different results. Still in this case, however, the differences between the results are within the error bars due to measurement noise. We conclude that, when applied to FORUM and IASI-NG missions, the two methods are equivalent from the accuracy point of view.

In recent years, the availability of a large amount of data has stimulated the use of synergistic approaches to gain the largest amount of information from measurements provided by a great number of satellite missions. Two classes of strategies are widely used to combine multiple sets of independent retrievals of the same atmospheric target: the synergistic retrieval (SR) and the a posteriori combination of the retrieved products. The Complete Data Fusion (CDF) algorithm was presented as a new a posteriori method to combine independent measurements of the same vertical profile of an atmospheric parameter into a single estimate for a concise and complete characterization of the atmospheric state. The CDF is able to reduce the complexity of managing high volumes of data and to improve their quality with respect to the operational outcome of individual instruments. FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment has been selected to be the ninth Earth Explorer mission of the European Space Agency (ESA) and it is scheduled for launch on a polar orbiting satellite in 2027. The core instrument will be a Far-InfraRed (FIR) spectrometer measuring, at nadir, the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum, from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a spectral resolution of 0.5 cm-1. FORUM will fly in loose formation with the MetOp-SG satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). Since IASI-NG will measure the Middle-InfraRed (MIR) part of the upwelling atmospheric spectrum (from 645 to 2760 cm-1), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements. The Level 2 products of FORUM include the water vapour profile, surface spectral emissivity in dry regions, cloud parameters and spectral fluxes. In this work, we present the results obtained from the application of two synergistic approaches, the SR and the CDF, to IASI-NG and FORUM synthetic measurements. We carried out two sets of test retrievals, based on synthetic clear-sky measurements, emulating an idealized situation in which both FORUM and IASI-NG measure, either with perfect matching or with a realistic mismatch, for 900 times, the same area over the Antarctic Plateau surface covered by coarse snow. The retrieval products considered in this study include the profiles of water vapour and temperature, surface temperature and spectral emissivity from 100 to 2300 cm-1. To characterize the quality of synergistic and fused products we evaluate their number of degrees of freedom as well as their total errors from the statistics of the differences between retrieved and true state parameters. Finally we compare the results obtained by the two methods by considering the statistics of the differences between synergistic and fused products. We find that in case of perfectly matching measurements, SR and CDF provide results that differ by less than 1/10 of their associated noise retrieval error. If a realistic mismatch (both in time and space) between the measurements is considered, the two methods provide more different results but with differences that are still within the retrieval error. The results obtained from the two methods are compared and, for both of them, the effect of the mismatch on the synergistic products is quantified.

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, selected for the ESA's 9th Earth Explorer (EE9) mission, will be launched in 2026. For the first time, spectrally resolved radiance observations covering the Far InfraRed (FIR) band from 100 to 667 cm ?¹ with global coverage and with high spectral resolution and radiometric accuracy will be available. This spectral region is highly sensitive to upper-tropospheric-lower-stratospheric (UTLS) water vapor concentration. One of the tools used to support the FORUM proposal during the EE9 competition, and previously, for the data analysis of the REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) balloon observations, is KLIMA (Kyoto protocoL Informed Management of the Adaptation) retrieval code. KLIMA analysed the synthetic FORUM observations in clear sky providing simultaneously the vertical profile of the minor atmospheric gases (such as the water vapour), atmospheric temperature profile, surface temperature, and surface emissivity. During the EE9 phase A selection, KLIMA was employed in various ESA studies such as: FORUM-req for the assessment of the FORUM mission requirements definition; FORUM-E2E for the validation of the End-to-End Simulator (E2ES); FIRMOS for the analysis of the ground-based observations of the FORUM prototype FIRMOS (Far-Infrared Radiation Mobile Observation System). Recently, the KLIMA code was used in the context of the ASI FORUM-science project for the generation of gases optical thicknesses used by the fast code ?-FORUM. In this framework, KLIMA has been upgraded to retrieve also the concentration of the isotopologues of the atmospheric gases. In this work, we will show the assessment of the performance of the retrieval of water vapour isotopologues using the spectral coverage and radiometric accuracy of the FORUM observations, and the comparison with the results obtained when only the middle-infrared information is used. Results both on single measurement and averaged measurements will be shown to evaluate the effect of the radiometric accuracy of the FORUM observations.

In 2019, the Italian Space Agency (ASI) started a new project entitled FORUM - scienza (FORUM science). One of the project objectives is to improve and share, within the interested National scientific community, the models needed for interpretation and analysis of the measurements of the forthcoming FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment. FORUM will be the ninth Earth Explorer mission of the European Space Agency (ESA), scheduled for launch on a polar satellite in 2026. The core instrument will be a Fourier Transform Spectrometer covering the spectral region from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a full-width-half-maximum spectral resolution of 0.5 cm-1. The spectrometer will measure the Earth's upwelling spectrum in the nadir-looking geometry. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). The MIR (645 to 2760 cm-1) upwelling atmospheric spectrum measured by IASI-NG will complement the FORUM FIR spectrum, providing an unprecedented spectral 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, 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 parameters. Among the models improved in the frame of the FORUM - scienza project, ?-FORUM is a fast parametric radiative transfer code with the capability of simulating the upwelling spectrum, from 50 to 2810 cm-1, with sufficiently high spectral resolution (0.01 cm-1), in all sky conditions. Along with the spectral radiance, the model computes its Jacobians with respect to relevant atmospheric, cloud and surface parameters. The characteristics and the accuracy figures of the ?-FORUM model are presented by Masiello et al., within this conference. Based on ?-FORUM, we have built a fast and flexible Bayesian inversion model. This model includes the possibility of performing the synergistic inversion of both FORUM and IASI-NG measurements and to apply various types of constraints in the inversion, such as Optimal Estimation and / or Tikhonov regularization. Profile retrieval grids, together with the set of parameters to be retrieved are selected via input files. Currently, the following sets of parameters can be retrieved: temperature, water vapour and ozone profiles, surface temperature and surface spectral emissivity (at a selectable set of wavenumbers). The program also offers the possibility to minimize the effect of possible frequency scale errors of the spectrum by retrieving a frequency scale factor. The possibility to retrieve profiles of ice- and liquid- water contents and of droplet- and ice- effective dimensions will be implemented in the near future. In this work we illustrate the structure of the developed retrieval model, its performance, and the results of the first tests we made to optimize the retrieval setup in clear-sky conditions.

The Earth's Outgoing Longwave Radiation (OLR) spectrum from 100 to 1600 cm-1 that will be measured by FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring), the ninth Earth Explorer mission of ESA, planned for the launch in 2026, can be exploited to retrieve the longwave spectral fluxes. In this study, we consider the approach that calculates the spectral flux from the atmospheric state retrieved from the spectral radiance measurements. The atmospheric state (mainly, vertical profiles of humidity and temperature, and ozone column), cloud and surface properties are retrieved using an inversion model based on a line-by-line radiative transfer code with the capability of simulating the multiple scattering. The retrieved parameters are then used to simulate the hemispherical spectral emission at the top-of-atmosphere that can be angularly integrated to give the spectral flux, potentially extending the spectral range to include the whole OLR. The radiance is assumed to be homogeneous in azimuth and the integral over the zenith angle is solved using a Gaussian quadrature. If the spectral radiance measurement contains most of the emitted energy, as in the case of FORUM, the error due to the conversion of radiance to flux can be very small. We apply this approach to one of the few existing set of measurements covering the wide-spectral range of FORUM in conditions similar to space observations, i.e. from a stratospheric balloon platform. This measurement was performed in 2005 from Teresina, Brazil, by the Radiation Explorer in the Far InfraRed - Prototype for Applications and Development (REFIR-PAD) instrument that was operated to measure at nadir the spectral radiance from 100 to 1400 cm-1 with a spectral resolution of 0.5 cm-1. The flight lasted approximately 8 hours acquiring measurements from an average floating altitude of 34 km in clear sky conditions. From these measurements, the vertical profiles of humidity and temperature and the surface temperature were retrieved using a few inversion codes based on forward models developed at our own premises (KLIMA, SACR, and ?-FORUM). Spectral fluxes were then computed via a quadrature approach. From spectral fluxes, band integrated fluxes can be finally calculated and compared with the results from the Rapid Radiative Transfer Model (RRTM). We discuss the advantages and the disadvantages of these methods and analyse the limits of their applicability.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment will be the ninth Earth Explorer mission of the European Space Agency (ESA), it is scheduled for launch on a Polar satellite in 2026. The FORUM core instrument will be a Fourier Transform Spectrometer covering both the Far-Infrared (FIR) and the Middle-InfraRed (MIR) spectral regions. It will measure, at nadir, the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum, from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a full-width-half-maximum spectral resolution of 0.5 cm-1. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). The MIR (645 to 2760 cm-1) upwelling atmospheric spectrum measured by IASI-NG will nicely complement the FORUM FIR spectrum, providing an unprecedented spectral 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, 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, that are essential to constrain atmospheric and surface parameter retrievals. In this study, we compare two alternative approaches to determine atmospheric and surface state parameters by exploiting simultaneously both FORUM and IASI-NG measurements. We examine the synergistic retrieval of state parameters from the simultaneous inversion of both measurements, and the a posteriori combination, via the Complete Data Fusion method, of the state parameters retrieved from the individual measurements. We perform test retrievals based on synthetic clear-sky measurements and characterize the quality of synergistic and fused products by means of their error evaluated both from the covariance matrix of the retrieval and from the statistics of the differences between retrieved and true state parameters. The retrieval products considered in this study include the profiles of water vapour and temperature, surface temperature and spectral emissivity from 100 to 2300 cm-1. As expected, we find that the exploitation of IASI-NG measurements permits to improve significantly the quality of FORUM retrieval products. These improvements, however, are progressively lost as soon as the mismatch between the two measurements increases. While synergistic retrieval and Complete Data Fusion provide almost identical results for perfectly matching measurements, differences comparable to the noise error are observed in the presence of time- and / or space- mismatches. We discuss the advantages and disadvantages of each of the two methods and analyze the limits of their applicability.

In cold and dry conditions typical of high-latitude and high-altitude locations, a significant fraction of the far-infrared (FIR) emission from the surface can reach the top of atmosphere (TOA). Recent works examined the impact of FIR surface emissivity on climate model projections and highlighted the influence of surface spectral emissivity on the surface and TOA FIR radiation budgets of polar regions. As spectral measurements in the FIR are still scarce, the authors of these studies used theoretical modelling to generate representative surface spectral emissivities. FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) is expected to be launched in 2026, and its core instrument will measure across Earth's FIR part of the electromagnetic spectrum. One of the key objectives of the mission is to obtain an extensive database of surface emissivity in the FIR. During the mission phases A and B1, the Far-Infrared Radiation Mobile Observation System (FIRMOS), a Fourier transform spectrometer, was built as an instrument demonstrator. In the future FIRMOS may also contribute to FORUM validation from the ground and from stratospheric balloon platforms. In the end of 2018, FIRMOS was deployed for a two-month ground-based campaign on Mount Zugspitze (German Alps, 2962m a.s.l.) where it measured the downwelling atmospheric spectral radiance from 100 to 1000 cm -1 (10-100 µm) with a resolution of 0.3 cm -1 . During this campaign, upwelling snow surface emission spectra were also acquired with FIRMOS. A variety of 9 snow and ice samples were collected in the vicinity of the experiment site. The samples were characterised in terms of snow grain type, density (kg m -3 ), and specific surface area (SSA, m 2 kg -1 ). The radiance measurements of the samples and of the atmosphere were inverted to retrieve spectral emissivity by means of an algorithm based on optimal estimation. This presentation describes the modelling of the inverse problem, the retrieval method and the preliminary results obtained.

For the preparation of the FORUM Earth Explorer mission, which will be launched by ESA in 2026, a mobile FTS prototype was developed to perform far-IR measurements of the atmospheric emission from ground-based sites and stratospheric bal- loons.

The Simultaneous Atmospheric and Cloud Retrieval (SACR) code is an algorithm comprising a forward and a retrieval model capable to simultaneously retrieve the main surface and atmospheric variables and of the cloud optical and microphysical properties, from infrared spectral radiance measurements. These atmospheric parameters are relevant in climate change studies. In particular, the characterization of cirrus cloud radiative properties represents a paramount goal, since this type of clouds modulates the incoming solar radiation and the thermal outgoing emission from the ground. Furthermore, cirrus clouds cover permanently more than 30% of the whole planet surface reaching 70% in the tropical regions. Despite their importance in the Earth Radiation Budget (ERB), cirri are difficult to detect, especially in polar regions, and their properties are still affected by large uncertainties. It has been recently recognized that the spectral region between 100 and 1600 cm -1 (6.2-100 ?m), and in particular the far infrared (FIR) region below 600 cm -1 , contains key information about the atmospheric water vapour and ice clouds. Despite this, there is a lack of spectrally resolved measurements covering the FIR region, either from air/space borne and from ground-based sensors. To cover this observational gap, the Far infrared Outgoing Radiation for Understanding and Monitoring (FORUM) mission was selected by the European Space Agency (ESA) as the 9th Earth Explorer (EE-9). For the first time from space, FORUM will acquire spectrally resolved measurements of the Earth emission. The procedure to retrieve relevant atmospheric parameters from FIR and MIR simulated spectral radiances (as those that will be measured by FORUM) is discussed in this work. Specifically, the parameters considered in the retrieval are: the water vapour and temperature profiles, the cloud optical depth, effective diameters of the particle distributions, the ice fraction in the presence of mixed phase clouds, the cloud position and the surface temperature. The performance of the retrieval is examined on the basis of a few test cases.

The Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission will provide a significant opportunity to verify and potentially improve the ability of global circulation models (GCMs) in simulating the Outgoing Longwave Radiation (OLR). Accurate simulation of the OLR is in fact crucial to better constrain the different radiative feedbacks. Planned for launch in 2026, FORUM will measure nadir-looking, spectrally resolved radiances of the Earth's emission spectrum at the top of the atmosphere (TOA) from 100 to 1600 cm-1 filling the existing observational gap of the far-infrared (FIR) region, from 100 to 667 cm-1. In addition, FORUM will fly in loose formation with IASI-NG, which will continue to cover the middle infrared range of IASI from 675 to 2760 cm-1. In anticipation of FORUM measurements, we aim at comparing IASI existing observations to synthetic radiances extracted from the EC-Earth GCM (version 3.3.2), a recent European model based on ECMWF's Integrated Forecasting System (IFS) for the atmosphere-land component and the ocean model NEMO, including sea ice (LIM2) and land surface components (1). Climate model outputs only provide simulated energy fluxes integrated over the whole Earth emission spectrum making difficult the detection of biases and the identification of possible compensation errors in the estimation of the OLR. Conversely, comparing simulated to observed spectra, allows to point out the potential model criticalities in specific spectral bands which contain the signatures of particular climate variables. In order to extract simulated spectra from the climate model, EC-Earth has been equipped with COSP, a simulator package able to map the model state into synthetic observations from different satellite-borne active (CloudSat (radar) and CALIPSO (lidar)) and passive (ISCCP, MISR and MODIS) (2) sensors. We have further developed the package by implementing inside COSP the radiative transfer model ?-FORUM, a monochromatic code able to reproduce synthetic radiances in the Far-Infrared and Mid-Infrared regions compatible with future FORUM and existing IASI observations (3). Due to the high computation cost of the operation, the efficiency of the EC-Earth model equipped with the new COSP module has been improved by modifying the ?-FORUM original code structure and by reducing the original resolution of the model. Therefore, simulations provided by EC-Earth model equipped with the new COSP + ?-FORUM module have been performed with prescribed sea surface temperature and sea-ice cover every 6 hours, over a timeframe consistent with the availability of IASI METOP-A L1C data, from 2007 to 2016. The comparison between nadir radiances simulated by the EC-Earth climate model and the climatology built from ten years of IASI observations represents a very high confidence test for the direct verification and improvement of the GCM avoiding potential uncertainties related to application of retrieval methods. The same approach could be extended to other climate models and, in the near future, it will involve FORUM measurements for a comprehensive analysis of the climate model ability in reproducing the whole Earth emission spectrum. References 1. Hazeleger, Wilco, et al. "EC-Earth: a seamless earth-system prediction approach in action." Bulletin of the American Meteorological Society 91.10 (2010): 1357-1364. 2. Bodas-Salcedo, Alejandro, et al. "COSP: Satellite simulation software for model assessment." Bulletin of the American Meteorological Society 92.8 (2011): 1023-1043. 3. Amato, Umberto, et al. "Corrigendum to: "The sigma-IASI code for the calculation of infrared atmospheric radiance and its derivatives" [Environmental Modelling & Software (2002) 17 (7) 651-667]." Environmental Modelling and Software 18.1 (2003): 97

During the last 30 years, many works have focused on the determination of the CO abundance in Titan's atmosphere, but no measurement above 300km has been done yet, due to the faint emission of CO in the upper atmosphere. Nevertheless, such a study is particularly awaited as a conrmation of photochemical models that predict a uniform volume mixing ratio of CO in the whole atmosphere. Moreover, given that CO is the main atmospheric reservoir of oxygen, its actual abundance has implications on the origins of Titan's atmosphere. In this work, we analyse a set of Cassini VIMS daytime limb observations of Titan at 4.7 m, which is dominated by solar-pumped non-LTE emission of CO rovibrational bands. In order to study this non-LTE emission, we have developed a non-LTE model for CO vibrational levels. Retrieval of the CO concentration is performed following a bayesian approach and using the calculated non-LTE populations. The data set analysed is constituted by 45 limb scanning sequences - about two thousand spectra - obtained by VIMS in 2006 and 2007. CO relative abundance proles from 200 to 500km are obtained, for each set analysed. The average prole shows little variation with altitude ranging from 50 ppm at 200km to 60 ppm at 500 km. The measurements are however well compatible with a uniform volume mixing ratio prole of 55100 ppm, thus compatible with the prediction of photochemical models of a well-mixed vertical prole.

Atmospheric emissions of carbon tetrachloride (CCl4) are regulated by the Montreal Protocol due to its role as a strong ozone-depleting substance. The molecule has been the subject of recent increased interest as a consequence of the so-called mystery of CCl4, the discrepancy between atmospheric observations and reported production and consumption. Surface measurements of CCl4 atmospheric concentrations have declined at a rate almost 3 times lower than its lifetime-limited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl4 vertical and zonal distributions in the upper troposphere and lower stratosphere (including the photolytic loss region, 70–20 hPa), its trend, and its stratospheric lifetime using measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which operated onboard the ENVISAT satellite from 2002 to 2012. Specifically, we use the MIPAS data product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency.The CCl4 zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the stratosphere, with larger quantities in the troposphere and a monotonic decrease with increasing altitude in the stratosphere. MIPAS CCl4 measurements have been compared with independent measurements from other satellite and balloon-borne remote sounders, showing a good agreement between the different datasets.CCl4 trends are calculated as a function of both latitude and altitude. Negative trends of about −10 to −15 pptv decade−1 (−10 to −30 % decade−1) are found at all latitudes in the upper troposphere–lower stratosphere region, apart from a region in the southern midlatitudes between 50 and 10 hPa where the trend is positive with values around 5–10 pptv decade−1 (15–20 % decade−1). At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements (−10 to −13 pptv decade−1). For higher altitudes, the trend shows a pronounced asymmetry between the Northern and Southern hemispheres, and the magnitude of the decline rate increases with altitude. We use a simplified model assuming tracer–tracer linear correlations to determine CCl4 lifetime in the lower stratosphere. The calculation provides a global average lifetime of 47 (39–61) years, considering CFC-11 as the reference tracer. This value is consistent with the most recent literature result of 44 (36–58) years.

Earth's emitted energy in the wavelength range 4 ?m-100 ?m is commonly termed Outgoing Longwave radiation (OLR) and its accurate measurement is a first-priority requirement to assess the energy balance of the planet. Integrated broadband OLR fluxes provided by CERES energy balanced and filled (EBAF) dataset constitutes the standard reference to study the Earth's energy budget and consequently represents the target values of energy fluxes at top of atmosphere (TOA) for climate models tuning. Even if tuning operations guarantee a good agreement on a global basis between observed and simulated OLR at TOA, large regional biases are present all over the globe and a significant spread exists between different climate model simulations. The Far-Infrared Outgoing Radiation and Monitoring (FORUM) mission, planned for launch in 2026, will provide nadir-looking spectrally resolved radiance measurements of the Earth's emission spectrum at TOA from 100 to 1600 cm-1 (6.25 ?m-100 ?m) with a resolution better than 0.50 cm-1. For the first time FORUM measurements will fill the existing observational gap in the far-infrared (FIR) radiation range from 100 to 667 cm-1 (15 ?m-100 ?m) also providing spectrally resolved OLR fluxes (level 2 data product). In anticipation of FORUM measurements, we want to quantify the potential improvement of climate models in simulating OLR fluxes at TOA due to the future availability of spectrally resolved fluxes data from FORUM, extending to the Far InfraRed. At this stage, we focus on the analyses of simulations provided by EC-Earth (version 3.3.2), a new climate model developed by the joint effort of 27 research institutes of 10 European countries which has directly contributed to Coupled Model Intercomparison (CMIP) in its phases 5 and 6. The EC-Earth model is based on ECMWF's Integrated Forecasting System (IFS) for the atmosphere-land component and the ocean model NEMO, including sea ice (LIM2) and land surface components. Atmospheric profiles and information on the surface and on the clouds produced by EC-Earth simulations in realistic but simplified conditions are provided in input to SIGMA-IASI, a radiative transfer model able to calculate spectral radiances compatible with those that will be measured by FORUM. From these synthetic measurements, we estimate OLR fluxes by applying angular distribution models (ADM) of increasing complexity. The comparison between the OLR fluxes provided by the EC-Earth simulation and those calculated from synthetic FORUM measurements may allow to identify potential improvements for the parametric radiative module included in EC-Earth to estimate the OLR radiative fluxes.

International audience; In 1999, the first sprites were observed above European thunderstorms using sensitive cameras. Since then, Eurosprite campaigns have been conducted to observe sprites and other transient luminous events (TLEs), expanding into a network covering large parts of Europe and coastal areas. In 2009 through 2013, the number of optical observations of TLEs reached a peak of 2000 per year. Because of this unprecedented number of European observations, it was possible to construct a climatology of 8394 TLEs observed above 1018 thunderstorm systems and study for the first time their distribution and seasonal cycle above Europe and parts of the Mediterranean Sea. The number of TLEs per thunderstorm was found to follow a power law, with less than 10 TLEs for 801 thunderstorms and up to 195 TLEs above the most prolific one. The majority of TLEs were classified as sprites, 641 elves, 280 halos, 70 upward lightning, 2 blue jets and 1 gigantic jet. The climatology shows intense TLE activity during summer over continental areas and in late autumn over coastal areas and sea. The two seasons peak, respectively, in August and November, separated by March and April with almost no TLEs, and a relative minimum around September. The observed TLE activity, i.e. mostly sprites, is shown to be largely consistent with lightning activity, with a 1/1000 of observed TLE-to-lightning ratio in regions with most observations. The overall behaviour is consistent among individual years, making the observed seasonal cycle a robust general feature of TLE activity above Europe

Rationale: Complex immune dysregulation in interferon (IFN) and T cell response has been observed in human immunodeficiency virus (HIV-1)-infected patients as well as in coronavirus disease-2019 (COVID-19) patients. However, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2)/HIV-1 coinfection has been described in only few cases worldwide and no data are available on immunological outcomes in HIV-1-patients infected with SARS-CoV-2. Hence, this study aims to compare type I IFN response and T cell activation levels between a SARS-CoV-2/HIV-1-coinfected female patient and age-matched HIV-1-positive or uninfected women. Patient concerns: A 52-year-old woman diagnosed with SARS-CoV-2/HIV-1 coinfection, ten HIV-1-positive women and five age-matched-healthy individuals were enrolled in this study. Diagnoses: SARS-CoV-2 infection caused severe pneumonia in the second week of illness in HIV-1-positive patient under protease inhibitors. Chest high-resolution computed tomography images of the SARS-CoV-2/HIV-1-coinfected patient showed bilateral ground-glass opacities. Interventions: SARS-CoV-2/HIV-1-coinfected female patient under darunavir/cobicistat regimen received a 7-days hydroxychloroquine therapy. Analysis of IFNα/β mRNA levels and CD4 and CD8 T cell (CD38, human leukocyte antigen-DR [HLA-DR], CD38 HLA-DR) frequencies were performed by RT/real-time PCR assays and flow cytometry, respectively. Median relative difference (MRD) was calculated for each immunological variable. For values greater than reference, MRD should be a positive number and for values that are smaller, MRD should be negative. Outcomes: The severe pneumonia observed in SARS-CoV-2/HIV-1-positive patient under protease inhibitors was reversed by a 7-days hydroxychloroquine therapy. At the end of treatment, on day 7, patient reported resolution of fever, normalization of respiratory rate (14 breaths/min), and improved oxygen arterial pressure with a FiO2 of 30%. MRD values for IFNα/β and CD4 and CD8 T cells expressing CD38 and/or HLA-DR found in SARS-CoV-2-/HIV-1-coinfected woman were approximatively equal to 0 when refereed respectively to HIV-1-positive female patients [MRDs IFNα/β: median −0.2545 (range: −0.5/0.1); T cells: median −0.11 (range: −0.8/1.3)] and ≥ 6 when referred to healthy individuals [MRDs IFNα/β: median 28.45 (range: 15/41.9); T cells: median 10 (range 6/22)]. Lessons: These results indicate that SARS-CoV-2 infection in HIV-1-positive female patient was associated with increased levels of IFNα/β-mRNAs and T cell activation compared to healthy individuals.

We developed a Self-Adapting Constraint Retrieval Scheme (SACRS) to retrieve ozone profiles from nadir infrared satellite measurements. In this algorithm, the constraint is variable in altitude and adapted automatically for each individual measurement. The algorithm is tested on synthetic observations representing the future IASI-NG satellite observations and considering either ozonesonde measurements or chemistry-transport model ozone simulations to represent the true ozone (pseudo-reality). The ozone retrievals are evaluated mainly for the troposphere with a specific focus on the lower troposphere between the surface and 6 km. Compared to a previous algorithm based on a fixed constraint retrieval scheme (FCRS), the biases, correlation and error estimates are improved with the SACRS. The bias is reduced by 40% and the correlation coefficient increases from 0.72 to 0.80. The SACRS algorithm also leads to an enhanced sensitivity in the lower troposphere with degrees of freedom for signal up to 0.83, increased by 11% compared to the FCRS. The SACRS performs especially well where current algorithms usually fail, namely for polar and tropical air masses. The bias is reduced from 8.6% to 0.5% in the troposphere (surface-9 km) when considering polar cases and from 24.4% to 10.1% in the upper troposphere - lower troposphere column (12–18 km) in the tropics.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), a limb-viewing infrared interferometer, operated on-board the ENVISAT satellite from 2002 to 2012. In the past years a huge effort was performed in correcting the time dependent non-linear response of its detectors, in order to produce calibrated measurements stable in time. The new calibration strategy has been implemented into the latest version (V8) of the Level 1 processor. Also the Level 2 (L2) processor has been upgraded, the main improvements being the handling of horizontal gradients and the use of an upgraded spectroscopic database. Both new processors have been used to generate three Diagnostic Datasets. The new L2 datasets contain the vertical distributions of Temperature and the VMR of H2O, O3, HNO3, CH4, N2O, NO2, CFC-11, CFC-12, N2O5, ClONO2, HCFC-22, COF2, CF4, HCN, CCl4, OCS, CH3Cl, HDO, C2H2, C2H6, and COCl2. For each month of MIPAS lifetime, the data have been averaged in latitudinal bins at fixed pressure levels, and for each level, trends have been estimated using the same method described in Valeri et al. (2017). In this paper we will show some examples of the estimated trends, concentrating on reactive molecules like phosgene.

In November 2017 the European Space Agency selected FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) as one of the two instrument concepts to be developed further and to compete to be the ninth Earth Explorer mission. FORUM will be a Fourier Transform Spectrometer measuring the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum from satellite. In particular, the instrument will cover the spectral range from 100 to 1600 cm -1 (from 100 to 6.25 microns in wavelength), covering the Far InfraRed (FIR), between 100 and 667 cm -1 , which has never been done from space before. Idealized model simulations suggest that between one quarter and one third of the total clear-sky long-wave cooling of the Earth to space occurs within the FIR spectral region. An even larger fraction of this cooling is predicted to occur in the FIR under all-sky conditions, because the presence of clouds causes lower emitting temperatures, and hence a shift to longer wavelengths of the peak of the black-body curve. For this reason, a detailed study of the Earth's radiation budget requires accurate, frequent, and global-coverage measurements of the total upwelling irradiance. In this paper we show that FORUM measurements will permit to obtain a very accurate estimate of the upwelling FIR irradiance. The achieved accuracy will be much better than that achievable with state-of-the-art forward model calculations of the FIR spectrum, based on the water vapor distribution determined from operational Mid- Infra-Red measurements, such as those from the Infrared Atmospheric Sounding Interferometer (IASI) on Met-Op satellites. For simplicity reasons the presented study is limited to clear-sky. In cloudy conditions, due to the large uncertainties in cloud composition, forward model simulations are expected to be even less accurate.

MIPAS on ENVISAT performed almost continuous measurements of atmospheric composition for approximately 10 years, from June 2002 to April 2012. ESA processor, based on the algorithm ORM (Optimized Retrieval Model), originally designed for the Near Real Time analysis, is currently used for the reanalysis of the full MIPAS mission. Version 7 of the full mission data was released in 2016, but further improvements have been recently performed in ORM V8 to be used in next full mission reanalysis. For these latest releases (V7 and V8) L1 data corrected for reducing the instrumental drift are used.TheinstrumentaldriftisduetoMIPASphotometricdetectorsnonlinearitiesthatchangewithtimeduetothe ageing of the instrument. Numerous species are retrieved from MIPAS measurements. Among them, CCl4 has been recently studied. This species has received increasing interest due to the so called "mystery of CCl4", since it was found that its atmospheric concentration at the surface declines with a rate significantly smaller than its lifetime-limited rate. Indeed there is a discrepancy between the atmospheric observations and the estimated distribution based on the reported production and consumption. MIPAS products generated with Version 7 of the L2 ESA algorithm were used to estimate CCl4 distributions, its trend, and atmospheric lifetime in the upper troposphere / lower stratosphere (UTLS) region. The trends derived by these observations between 2002 and 2012 as a function of both latitude and altitude confirm the decline of atmospheric mixing ratios, in agreement with ground based observations. Stratospheric trend derived from the MIPAS data are non-uniform, with some positive trends even being found in the middle stratosphere, mainly at high altitudes in the Southern Hemisphere. The variability in stratospheric trends reflects the impact of variability in stratospheric transport on trace gases and their temporal evolution. In addition to CCl4, some preliminary results obtained with the latest version of the processor (V8), that performs the analysis of a larger number of species and takes into account horizontal inhomogeneities, will be shown.

The instrument FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring), has been selected to compete for the next Earth Explorer 9 mission. The primary goal of the FORUM mission is the characterization of the Outgoing Longwave Radiation in the far-infrared and in the middle infrared regions, from 100 to 1600 cm-1. During the preparatory stage of the proposal, several studies have been performed to address the capability of the proposed instrument to provide information on important climate variables like surface temperature and emissivity, vertical distributions of humidity, Temperature, trace gases, and ice clouds. Here we summarize the results of these sensitivity studies.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a mid-infrared limb emission sounder that operated on board the polar satellite ENVISAT from 2002 to 2012. The retrieval algorithm used by the European Space Agency to process MIPAS measurements exploits the assumption that the atmosphere is horizontally homogeneous. However, previous studies highlighted how this assumption causes significant errors on the retrieved profiles of some MIPAS target species.In this paper we quantify the errors induced by this assumption and evaluate the performances of three different algorithms that can be used to mitigate the problem. We generate synthetic observations with a high spatial resolution atmospheric model and carry out the retrievals with four alternative methods. The first assumes horizontal homogeneity (1-D retrieval), the second includes a model of the horizontal gradient of atmospheric temperature (1-D plus temperature gradient retrieval), the third accounts for an horizontal gradient of temperature and composition (1-D plus temperature and composition gradient retrieval), while the fourth is the full two-dimensional (2-D) inversion approach.Our results highlight that the 1-D retrieval implies errors that are significant for averages of profiles. Furthermore, for some targets (e.g. T, CH4 and N2O below 10 hPa) the error induced by the 1-D approximation also becomes visible in the individual retrieved profiles. The inclusion of any kind of horizontal variability model improves all the targets with respect to the horizontal homogeneity assumption. For temperature, HNO3 and CFC-11, the inclusion of an horizontal temperature gradient leads to a significant reduction of the error. For other targets, such as H2O, O3, N2O, CH4, the improvements due to the inclusion of an horizontal temperature gradient are minor. In these cases, the inclusion of a gradient in the target volume mixing ratio leads to significant improvements. Among all the methods tested in this work, the 2-D approach, as expected, implies the smallest errors for almost all the target parameters. This residual error of the 2-D approach is the smoothing caused by the retrieval grid, which is coarser than that of the atmospheric model.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a Fourier transform spectrometer that measured mid-infrared atmospheric limb emission spectra from July 2002 to April 2012 on board the polar-orbiting satellite ENVISAT. We have used MIPAS data to study the latitudinal variations of phosgene (COCl2 or carbonyl chloride) and, for the first time, its seasonal variation in the upper troposphere/lower stratosphere region (UTLS). Retrievals of phosgene were made using the 830–860 cm−1 region, corresponding to the ν5 bands of COCl2. Unfortunately, in that region, the ν4 band of CFC-11, which is much stronger than COCl2 ν5, hides the phosgene emission. In order to evaluate seasonality and latitudinal distribution of phosgene we have analysed all the measurements made by MIPAS on days 18 and 20 of each month of 2008 with the optimized retrieval model (ORM) recently upgraded with the multi-target retrieval technique and with the optimal estimation functionality to apply external constraints to the state vector. Average seasonal profiles of phosgene show an evident latitudinal variability with the largest values observed in the tropical regions (maximum ≈ 35 parts per trillion by volume (pptv) at about 300 hPa). In the midlatitude and polar regions, the volume mixing ratio (VMR) values do not exceed 30 pptv and the vertical distributions are less peaked. Our analysis highlights that COCl2 seasonal variability is fairly low, apart from the polar regions.

Improved line positions and intensities have been generated for the 7.6 µm spectral region of nitric acid. They were obtained relying on a recent reinvestigation of the nitric acid band system at 7.6 µm and comparisons of HNO3 volume mixing ratio profiles retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) limb emission radiances in the 11 and 7.6 µm domains. This has led to an improved database called MIPAS-2015. Comparisons with available laboratory information (individual line intensities, integrated absorption cross sections, and absorption cross sections) show that MIPAS-2015 provides an improved description of the 7.6 µm region of nitric acid. This study should help to improve HNO3 satellite retrievals by allowing measurements to be performed simultaneously in the 11 and 7.6 µm micro-windows. In particular, it should be useful to analyze existing MIPAS and IASI spectra as well as spectra to be recorded by the forthcoming Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG) instrument.

Far- to mid- infrared spectral radiances measured either from high altitude platforms or from ground can be processed to retrieve atmospheric vertical profiles and cloud parameters, variables particularly relevant in climate change studies. The retrieval requires a forward model with the capability of simulating the multiple scattering from cloud particles. The Discrete Ordinate Radiative Transfer (DISORT) offers this possibility, however, accurate simulations can be obtained only with a huge computational load. We developed a forward / retrieval model based on the two-streams δ-Eddington approximation, allowing much faster computations, while retaining good accuracy. The code, named SACR (Simultaneous Atmospheric and Clouds Retrieval), allows to retrieve simultaneously temperature and gas profiles, cloud micro-physical and geometrical parameters and surface temperature from vertical sounding observations. We illustrate the equations implemented in the SACR code, prove the self-consistency of the inversion and assess its forward model accuracy with a focus on the range from 200 to 1000 cm − 1 . The assessment is made by comparing the simulated spectral radiances to those computed by LBLDIS, a very accurate model integrating LBLRTM (Line By Line Radiative Transfer Model) and DISORT. For cloud particle sizes between 20 and 100 µm and optical depths between 0.1 and 10, our model shows biases smaller than 0.4 mW/(m2 sr cm − 1 ) in upwelling radiance simulations, and biases smaller than 0.3 mW/(m2 sr cm − 1 ) in downwelling radiance simulations. Depending on the spectral grid and on the number of atmospheric layers used, the SACR code is from 5 to 8 times faster than LBLDIS.

The FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission concept has been selected as one of the two candidates missions for the ESA's Earth Explorer 9 mission. FORUM will fly on a polar orbit with a ground footprint of 15 km diameter. The payload of FORUM is a Fourier Transform Interferometer (FTI) and an imager to be used to identify uniform scenes. The primary goal of the FORUM mission is the characterization of the Outgoing Longwave Radiation in the far-infrared and in the middle infrared regions, from 100 to 1600 cm-1. However, the measurement of the spectrally resolved emission of the Earth in that spectral region may be exploited to measure ice clouds, the vertical distribution of several trace gases (i.e. water, ozone) and the surface properties, especially over the polar regions where the relatively dry conditions enable to directly observe the Earth surface in the far-infrared. Moreover, the relatively high spectral resolution of FORUM will help in improving both the spectroscopy and the continuum knowledge in this so far underexploited spectral region. FORUM is currently undergoing phase A and several studies are ongoing to address the capability of the proposed instrument to provide information on important climate variables like surface temperature and emissivity, vertical distributions of humidity, temperature, trace gases, and ice clouds. Among them the FORUM-Req project is devoted to the consolidation of the instrumental requirements for the FTI and the imager in order to ensure that the science goals of FORUM will be met by the developed payload. In this paper we summarize the results obtained in the frame of the FORUM-Req project with the simulations of FORUM measurements in clear sky conditions. A set of representative atmospheric and surface conditions have been selected and used to simulate the radiation that FORUM will measure in its lifetime. These simulations have been used to study the accuracy with which FORUM will measure the UTLS distribution of trace gases and the polar surface properties and the sensitivity of the obtained results to systematic biases.

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been proposed within the framework of the 9th Earth Explorer program. It was selected for phase A studies by the European Space Agency (ESA) as suitable mission to be launched in close formation with IASI-NG. The instrument will measure the top of the atmosphere (TOA) outgoing radiance at nadir in a wide spectral range covering FIR (Far-Infrared) and TIR (Thermal Infrared) regions from 100 cm-1 to 1600 cm-1. The main target of the instrument, beside the measurement of the full spectrum of the Outgoing Longwave Radiation (OLR), is the estimate of water vapour in the upper troposphere-lower stratosphere (UTLS) region and clouds properties. However, FORUM is capable of measuring other variables such as ozone, temperature and surface emissivity. These targets are considered to become formal products and are examined in the retrieval tests performed in the phase A studies. In order to investigate the capability of FORUM to measure these variables, the error sources affecting the retrieval, both random and systematic, have to be quantified. While the random retrieval error, defined as in Rodgers et al. (2000), is directly linked to the instrumental noise, the systematic errors are due to both instrumental issues and to the approximations present in the retrieval procedure (i.e. quantities that are not fitted). Therefore the systematic error sources are a wide set of variables, like the Volume Mixing Ratios of interfering gaseous species, the water vapour continuum model, error in the spectroscopic data and instrumental characteristics. To estimate the impact of the systematic errors on the retrieval, we adopted the error spectra strategy already used by Dudhia et al. (2002) to assess the systematic errors for MIPAS/ENVISAT level 2 products. In this work, we will report both the used methodology and the results for the more relevant systematic errors affecting the retrieval of FORUM targets.

The primary goal of the FORUM mission is to measure the Earth's top-of-atmosphere emission spectrum in the 100 to 1600 cm-¹ spectral region covering the observational gap of the Far IR (100-667 cm-¹), which has never been observed from space in its entirety. This measurement will provide an improved understanding of the climate system by supplying, for the first time, most of the spectral features of the Far IR contribution to the Earth radiation budget, particularly focusing on the water vapour contribution related to the continuum absorption in the rotational band, cirrus cloud and ice/snow surface emissivity. This goal is achieved by measuring the spectrum with a nominal resolution of 0.3 cm-¹, and with an absolute calibration accuracy of 0.1 K, benchmarked against international standards. A precision of 0.4 mW/m² sr cm-¹ on the radiance will also allow the retrieval of the atmospheric state and cloud parameters with improved accuracy. A demonstration of the FORUM capabilities in improving the characterisation of the atmospheric radiative properties using the Far IR is shown by means of ground-based observations performed at high-altitude sites where the air is sufficiently transparent to sound the Far IR spectral features of the atmospheric emission. To this aim, two instruments are used: the REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) instrument, which has been deployed on the Antarctic Plateau, where it has been acquiring downwelling longwave radiation spectra continuously since 2012, and the FORUM prototype, which will be deployed at the Zugspitze summit at the end of 2018 - early 2019. Both instruments are Fourier transform spectrometers with an optical configuration of the interferometer similar to FORUM. They cover the Far IR spectral region down to 100cm-¹ and can be operated under all sky non-precipitating conditions. In this presentation, a review of the measurement requirements of the FORUM mission will be provided with a description of the measurement results obtained during the high-altitude Antarctic and Zugspitze campaigns.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a Fourier transform spectrometer designed for measuring the earth atmospheric emission at limb in the 685-2410 cm-1 range. It flew onboard the polar orbit ENVISAT satellite for approximately 10 years, from June 2002 to April 2012, performing measurements during both day and night with an unprecedented spectral resolution from space (0.025 cm-1 in the first part of the mission and 0.0625 cm-1 in the second part). MIPAS measurements on ENVISAT represent a unique database for the study of the vertical distribution and time variation of the atmospheric composition on a global scale. While the focus of the instrument is the study of the chemistry and the physics of the stratosphere, also information on constituents in the upper troposphere has been derived. The ESA products include more than 20 minor atmospheric constituents [1]. The retrieval of the vertical profiles of atmospheric constituents is performed by fitting the observations to a full-physics model taking into account emission and absorption of all molecules. As a consequence the possibility of performing the retrievals is directly linked to the availability of the spectroscopic parameters and the quality of the retrieval depends on their quality. Indeed, in the estimation of the total error budget of the retrieved profiles the error due to the spectroscopic database is often one of the largest ones. The need of exploiting the information contained in the measurements of MIPAS, as well as of other instruments operating in the same spectral region like ACE-FTS, has driven studies for improving spectroscopic parameters in the middle infrared spectral region. MIPAS measurements provided not only the motivation, but also the guideline and the validation of the spectroscopic studies. Thanks to the their high spectral resolution and broadband coverage, the differences between observations and simulations of MIPAS measurements have been used to detect inconsistencies and validate subsequent improvements in the spectroscopic database (obtained with laboratory measurements and new calculations). In some cases MIPAS measurements have been used to convert the relative line intensities of a given spectral band to absolute intensities by comparing the retrieved profiles from MIPAS radiances from two different regions covering the two different spectral bands [2]. In this talk we will show how MIPAS measurements were used to validate the most recent improvements in the spectroscopic database of HNO3 and COCl2.

In November 2017 the European Space Agency selected FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) as one of the two instrument concepts to be developed further and to compete to be the ninth Earth Explorer mission. FORUM will be a Fourier Transform Spectrometer measuring the entire far-infrared part of the Earth upwelling spectral radiance. In particular, the instrument will measure the spectral range from 100 to 1600 cm -1 (from 100 to 6.25 microns in wavelength), covering the Far InfraRed (FIR), between 100 and 667 cm -1 , which has never been measured from space before. The infrared radiation emitted by the Earth to space is affected by water vapor and cirrus clouds, which, in turn, play key roles in driving the Earth's temperature. FORUM measurements will provide new insight not only on the outgoing longwave radiation but also on the water vapor and cirrus clouds distribution over the globe. Therefore FORUM will improve our understanding of the greenhouse effect and will significantly contribute to the accuracy of climate change assessments that are the basis for political decisions. Although the main foci of the mission are water vapor and cirrus clouds, FORUM FIR measurements will also provide information on the vertical distribution of some additional minor atmospheric constituents, such as O 3 , HNO 3 , CH 4 and N 2 O. Based on simulated observations, in this paper we analyze the accuracy and the spatial resolution of these additional FORUM products. Moreover, the possibility to infer the total column of further gases that contribute to the spectrum with features significantly below the measurement noise level will be analyzed.

Atmospheric emissions of Carbon tetrachloride CCl4 are regulated by the Montreal Protocol due to its role as a strong ozone-depleting substance. The molecule has been the subject of recent increased interest as a consequence of the so called ``mystery of CCl4,'' the discrepancy between atmospheric observations and reported production and consumption. Surface measurements of CCl4 atmospheric concentrations have declined at a rate almost three times smaller than its lifetime-limited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl4 vertical and zonal distributions in the upper troposphere and lower stratosphere (including the photolytic loss region, 70–20 hPa), its trend, and its stratospheric lifetime using measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which operated onboard the ENVISAT satellite from 2002 to 2012. Specifically, we use the MIPAS data product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency. The CCl4 zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the stratosphere, with larger quantities in the troposphere and a monotonic decrease with increasing altitude in the stratosphere. In the troposphere, the largest concentrations are observed at the latitudes of major industrial countries (20/50°N). The good agreement we find between MIPAS CCl4 and independent measurements from other satellite and balloon-borne remote sounders proves the reliability of the MIPAS dataset. CCl4 trends are calculated as a function of both latitude and altitude. Negative trends are found at all latitudes in the upper-troposphere / lower-stratosphere region, apart from a region in the Southern mid-latitudes between 50 and 10 hPa where the trend is positive. At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements. For higher altitudes, the trend shows a pronounced asymmetry between Northern and Southern Hemispheres, and the magnitude of the decline rate increases with altitude. At 50 hPa the decline is about 30–35 %/decade, close to the lifetime-limited trend. We use a simplified model assuming tracer-tracer linear correlations to determine CCl4 lifetime in the lower stratosphere. The calculation provides a global average lifetime of 46(38–60) years considering CFC-11 as the reference tracer. This value is consistent with the most recent literature result of 44(36–58) years.

The MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument on the Envisat (Environmental satellite) satellite has provided vertical profiles of the atmospheric composition on a global scale for almost ten years. The MIPAS mission is divided in two phases: the full resolution phase, from 2002 to 2004, and the optimized resolution phase, from 2005 to 2012, which is characterized by a finer vertical and horizontal sampling attained through a reduction of the spectral resolution. While the description and characterization of the products of the ESA processor for the full resolution phase has been already described in previous papers, in this paper we focus on the performances of the latest version of the ESA (European Space Agency) processor, named ML2PP V6 (MIPAS Level 2 Prototype Processor), which has been used for reprocessing the entire mission. The ESA processor had to perform the operational near real time analysis of the observations and its products needed to be available for data assimilation. Therefore, it has been designed for fast, continuous and automated analysis of observations made in quite different atmospheric conditions and for a minimum use of external constraints in order to avoid biases in the products. The dense vertical sampling of the measurements adopted in the second phase of the MIPAS mission resulted in sampling intervals finer than the instantaneous field of view of the instrument. Together with the choice of a retrieval grid aligned with the vertical sampling of the measurements, this made ill-conditioned the retrieval problem of the MIPAS operational processor. This problem has been handled with minimal changes to the original retrieval approach but with significant improvements nonetheless. The Levenberg-Marquardt method, already present in the retrieval scheme for its capability to provide fast convergence for nonlinear problems, is now also exploited for the reduction of the ill-conditioning of the inversion. An expression specifically designed for the regularizing Levenberg-Marquardt method has been implemented for the computation of the covariance matrices and averaging kernels of the retrieved products. The regularization of the Levenberg-Marquardt method is controlled by the convergence criteria and is deliberately kept weak. The resulting oscillations of the retrieved profile are a posteriori damped by an innovative self-adapting Tikhonov regularization. The convergence criteria and the weakness of the self-adapting regularization ensure that minimum constraints are used and the best vertical resolution obtainable from the measurements is achieved in all atmospheric conditions. Random and systematic errors, as well as vertical and horizontal resolution are compared in the two phases of the mission for all products, namely: temperature, H2O, O3, HNO3, CH4, N2O, NO2, CFC-11, CFC-12, N2O5 and ClONO2. The use in the two phases of the mission of different optimized sets of spectral intervals ensures that, despite the different spectral resolutions, comparable performances are obtained in the whole MIPAS mission in terms of random and systematic errors, while the vertical resolution and the horizontal resolution are significantly better in the case of the optimized resolution measurements. © Author(s) 2013., This work has been performed under the ESA study >Support to MIPAS Level 2 product validation>, contract ESA-ESRIN no. 21719/08/I-OL. The authors are grateful to the Astrium team that developed the industrial prototype ML2PP using the ORM code as reference, to Thorsten Fehr and Rolf von Kulhmann for the coordination of the MIPAS Quality Working Group activities and to Michael Kiefer for the work done within the MIPAS Quality Working Group.