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On the East Antarctic Plateau, in winter, rapid warming events originated by the advection of warm, moist air from lower latitudes, cause the disruption of the stable thermal structure of the atmosphere, and can be linked to the warming of the Plateau region itself. Continuous monitoring of these events can shed light on temperature trends in East Antarctica, trends which are still not clearly defined in terms of origin and amount.Since the main mechanism acting in the warming events is the strong increase in cloud cover linked to the higher water content of the advected air, for a systematic monitoring of warming phenomena a simultaneous detection of water vapor vertical profile and cloud properties is needed. These two tasks can be both performed through the analysis of spectrally resolved atmospheric downwelling emitted radiances.The REFIR (Radiation Explorer in the Far Infrared) Fourier transform spectroradiometer was installed at Concordia station, in the Dome C region of the Antarctic Plateau, in December 2011, and it has been performing continuous measurement since then. REFIR measures the downwelling atmospheric radiance in the 100-1500 cm-1 (6.7-100 µm) spectral interval, with a resolution of 0.4 cm-1, and with a repetition rate of about 10 minutes. The measured spectral interval extends from the far infrared, which includes the water vapor rotational band, to the atmospheric window region (8-14 µm), which provides information about the radiative effects of clouds.A dedicated inversion code was developed to retrieve vertical profiles of water vapor and temperature from the measured emission spectra. The retrieved profiles allow for the monitoring of the evolution of the vertical structure of the troposphere on a 10 minutes timescale, whereas the spectral radiance itself provides, in a more direct way, information on the cloud cover. Therefore, the dataset produced by the REFIR instrument allow us to detect and obtain statistics about warming events in the Dome C region.

Cloud identification from satellites is considerably challenging in polar environments due to the similar radiative properties of surface and ice clouds, making it difficult to detect and distinguish cloud features. CIC (Cloud Identification and Classification) is a machine learning algorithm adopted as the official software in the ESA Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) (Palchetti et al., 2020) End2End simulator (FE2ES). CIC is based on Principal Component Analysis and performs cloud detection and multi-scene classification. It is adaptable to every type of sensor and is particularly suitable when a small number of elements are available for the Training Set. Assessment studies have already been conducted to evaluate the performances of the algorithm in multiple conditions. In Maestri et al. (2019), CIC was applied to simulated radiance all over the globe, while Magurno et al. (2020) used the algorithm to analyze airborne interferometric spectra. Finally, in Cossich et al. (2021) the algorithm was tested on downwelling radiances collected at Dome-C in Antarctica. In this work, CIC is applied to high spectrally resolved data taken from ground and, for the first time, from satellites. Ground-based data are collected by the REFIR-PAD sensor (Di Natale et al., 2020), covering the far and mid-infrared part of the spectrum. Collocated satellite data are measured by IASI (Infrared Atmospheric Sounding Interferometer) which collects upwelling radiance between 3.4 and 15.5 μm. The period under study spans from 2012 to 2022. CIC results applied to ground-measured spectra are compared to IASI’s L2 classification products. Large discrepancies between the two classifications are observed, indicating an overestimation of the cloud occurrence in case of IASI. A verification is obtained using collocated ground-based LIDAR measurements, which are available for subsets of the collocated radiances. Finally, the CIC algorithm is trained with a subset of IASI data collocated with REFIR-PAD and LIDAR measurements. The training set is defined also with the help of the Advanced Very High Resolution Radiometer (AVHRR) on board of MetOp satellites. The AVHRR has 1 km resolution (at the nadir) and its collocated measurements are used to evaluate the scene homogeneity in the satellite field of view. Statistical analyses are then performed on IASI spectra using the CIC classification. Results indicate a much better agreement with ground-based data, improving the cloud occurrence provided in IASI L2 products.

The Far-Infrared Radiation Mobile Observation System (FIRMOS) is a Fourier transform spectroradiometer developed to support the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) satellite mission by validating measurement methods and instrument design concepts, both in the laboratory and in field campaigns. FIRMOS is capable of measuring the downwelling spectral radiance emitted by the atmosphere in the spectral band from 100 to 1000 cm-1 (10–100 µm in wavelength), with a maximum spectral resolution of 0.25 cm-1. We describe the instrument design and its characterisation and discuss the geophysical products obtained by inverting the atmospheric spectral radiance measured during a campaign from the high-altitude location of Mount Zugspitze in Germany, beside the Extended-range Atmospheric Emitted Radiance Interferometer (E-AERI), which is permanently installed at the site. Following the selection of clear-sky scenes, using a specific algorithm, the water vapour and temperature profiles were retrieved from the FIRMOS spectra by applying the Kyoto protocol and Informed Management of the Adaptation (KLIMA) code. The profiles were found in very good agreement with those provided by radiosondes and by the Raman lidar operating from the Zugspitze Schneefernerhaus station. In addition, the retrieval products were validated by comparing the retrieved Integrated Water Vapour values with those obtained from the E-AERI spectra. Finally, we found that the trends for the temperature, and the water vapour profiles over time were in good agreement with those provided by ERA5 reanalysis.

Statistics on the occurrence of clear skies, ice clouds, and mixed-phase clouds over Concordia Station, in the Antarctic Plateau, are provided for multiple timescales and analyzed in relation to simultaneous meteorological parameters measured at the surface. Results are obtained by applying a machine learning cloud identification and classification (CIC) code to 4 years of measurements between 2012–2015 of downwelling high-spectral-resolution radiances, measured by the Radiation Explorer in the Far Infrared – Prototype for Applications and Development (REFIR-PAD) spectroradiometer. The CIC algorithm is optimized for Antarctic sky conditions and results in a total hit rate of almost 0.98, where 1.0 is a perfect score, for the identification of the clear-sky, ice cloud, and mixed-phase cloud classes. Scene truth is provided by lidar measurements that are concurrent with REFIR-PAD. The CIC approach demonstrates the key role of far-infrared spectral measurements for clear–cloud discrimination and for cloud phase classification. Mean annual occurrences are 72.3 %, 24.9 %, and 2.7 % for clear sky, ice clouds, and mixed-phase clouds, respectively, with an inter-annual variability of a few percent. The seasonal occurrence of clear sky shows a minimum in winter (66.8 %) and maxima (75 %–76 %) during intermediate seasons. In winter the mean surface temperature is about 9 ∘C colder in clear conditions than when ice clouds are present. Mixed-phase clouds are observed only in the warm season; in summer they amount to more than one-third of total observed clouds. Their occurrence is correlated with warmer surface temperatures. In the austral summer, the mean surface air temperature is about 5 ∘C warmer when clouds are present than in clear-sky conditions. This difference is larger during the night than in daylight hours, likely due to increased solar warming. Monthly mean results are compared to cloud occurrence and fraction derived from gridded (Level 3) satellite products from both passive and active sensors. The differences observed among the considered products and the CIC results are analyzed in terms of footprint sizes and sensors' sensitivities to cloud optical and geometrical features. The comparison highlights the ability of the CIC–REFIR-PAD synergy to identify multiple cloud conditions and study their variability at different timescales.

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.

Selected case studies of precipitating ice clouds at Dome-C (Antarctic Plateau) are used to test a new approach for the estimation of ice cloud reflectivity at 24 GHz (12.37 mm of wavelength) using ground-based far infrared spectral measurements from the REFIR-PAD Fourier transform spectroradiometer and backscattering/depolarization lidar profiles. The resulting reflectivity is evaluated with the direct reflectivity measurements provided by a co-located micro rain radar (MRR) operating at 24 GHz, which is able to detect falling crystals with large particle size, typically above 500 μm. To obtain the 24 GHz reflectivity, we used the particle effective diameter and the cloud optical depth retrieved from the far infrared spectral radiances provided by REFIR-PAD and the tropospheric co-located backscattering lidar to calculate the modal radius and the intercept of the particle size distribution. From these, the theoretical reflectivity at 24 GHz is obtained by integrating the size distribution over different microwave cross sections for various habit crystals provided by Eriksson et al. (2018) databases. From the comparison with the radar reflectivity measurements, we found that the column-like habits and the plates/columnar crystal aggregates show the best agreement with the MRR observations. The presence of (hexagonal) columns is confirmed both by the presence of 22° solar halos, detected by the HALO-CAMERA, and by the crystal images taken by the ICE-CAMERA, operating in proximity of REFIR-PAD and the MRR. The average crystal lengths obtained from the retrieved size distribution are also compared to the ones estimated from the ICE-CAMERA images. The agreement between the two results confirms that the retrieved parameters of the particle size distributions correctly reproduce the observations.

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.

Downwelling radiance spectra in the 100-1500 cm-1 interval, measured by the Radiation Explorer in the Far Infrared-Prototype for Applications and Development (REFIR-PAD) spectroradiometer on the Antarctic plateau since 2012, are ingested by an automatic machine learning algorithm, named cloud identification and classification (CIC), to detect and classify the Antarctic clouds. The CIC algorithm is a modified version of the one chained in the End-2-End Simulator (EES) of the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, the next ESA 9th Earth Explorer. CIC sits at the center of the decision tree of the FORUM EES for retrieving clear-sky atmospheric products or cloud properties. Co-located lidar measurements are exploited to define training sets composed of radiance spectra in presence of clear sky, ice clouds or mixed-phase clouds. The CIC is initially tested on a controlled verification subset for optimization. It is demonstrated that the information content in the far infrared (FIR) part of the spectrum is critical for the improvement of the performances of the algorithm to identify thin clouds and for cloud phase classification. A test set of 1726 spectra is then used to estimate the classification error. Unprecedented cloud occurrence statistics concerning more than 4 years of data are provided for multiple time scales and related to meteorological parameters such as surface air temperature and wind direction. The results indicate a clear sky mean annual occurrence of 72.3%, while ice and mixed-phase clouds are observed in 24.9% and 2.7% respectively, with an inter-annual variability of a few percent. The seasonal occurrence of clear sky shows a minimum in winter (66.8%) and maxima (75-76%) during intermediate seasons. In austral winter the mean surface temperature is about 9°C colder in clear conditions than when ice clouds are present. Mixed-phase clouds are observed only in the warm season (November-March). In austral summer they amount to more than one third of total observed clouds. Their occurrence is correlated with warmer surface temperatures. In the summer, the mean surface air temperature is about 5°C warmer when clouds are present than in clear sky conditions. A comparison of the CIC classification with available satellite Level-2 (L2) and Level-3 (L3) products, is provided. Passive (Infrared Atmospheric Sounding Interferometer - IASI, and Moderate Resolution Imaging Spectroradiometer - MODIS), and active (Cloud-Aerosol LiDAR with Orthogonal Polarization - CALIOP, and the Cloud Profiling Radar - CPR) sensors are considered. For selected case studies, a direct comparison between co-located L2 satellite products and parameters retrieved from ground-based observations (REFIR-PAD, lidar) is performed. In case of cloudy scenes, retrieved cloud parameter are used to simulate FORUM like observations and to evaluate the impact of the additional FIR part of spectrum in satellite cloud retrievals from infrared passive measurements. For L3 satellite products, the monthly gridded data are used for comparison. The differences observed among the considered products and the CIC results are analysed in terms of footprint sizes and sensors' sensitivities. The comparison highlights the ability of the CIC/REFIR-PAD to identify multiple cloud conditions from high spectral resolution radiances.

Since December 2011 the Radiation Explorer in the Far Infrared (REFIR) Fourier trasform spectroradiometer has been operating continuously from the Italian-French station Concordia, in the Dome C region, Antarctica, providing a decade long dataset of spectrally resolved atmospheric downwelling radiances in the mid/far-infrared range.In 2016 The Dome C Tropospheric Observatory (DOCTOR) project was established with the aim to recalibrate and reorganize the full time series of REFIR spectra in order to provide a homogeneous dataset, and to make it available to the scientific community.A further objective of the DOCTOR project has been to integrate the REFIR spectroradiometer with a Lidar sensor to provide coincident, colocated measurements of tropospheric backscatter profiles.The downwelling radiance spectra are processed with a retrieval code which is capable to provide vertical profiles of tropospheric temperature and water vapor. The availability of coincident backscatter profiles permits to improve the performance of the retrieval in cloudy sky conditions, providing the vertical structure of clouds which is not easily inferred from the spectra alone.The resulting observation repository will provide a relevant source of information about tropospheric trends in a region, the East Antarctic plateau, which is sparsely covered by ground-based measurements.

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.

In this paper a retrieval procedure of cirrus ice properties, based on the optimal estimation and the Levenberg-Marquardt iterative formula, is extended with a suitable non linear transformation of the cloud state vector parameters to include the crystal habit distribution in addition to the effective diameter and the optical depth. The cloud optical coefficients are linearly combined using different crystal shapes (solid columns, droxtals, hollow columns, bullet rosettes, aggregates and plates) to build up two different mixtures of habits to be used in the retrieval. This approach allows us to retrieve the coefficients of the habit crystals inside cirrus clouds by using upwelling far infrared spectral radiance measurements. The retrieval capability is shown by performing self-consistency tests using the Simultaneous Atmospheric and Cloud Retrieval (SACR) code and a set of 240 synthetic diverse observations in cloudy-sky generated by using realistic atmospheric scenarios (for mid-latitudes, tropics and poles) and the noise anticipated for the new far infrared sensor Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM), which is planned to be launched as the ninth Earth Explorer (EE-9) by the European Space Agency (ESA) in 2026. The sensitivity of the retrieved crystal habit coefficients to parameters that are not retrieved, such as the surface temperature and emissivity, the cloud top and bottom heights, the ice fraction, the water vapour and temperature profiles, together with the biases caused by the cloud vertical and horizontal inhomogeneity, were also assessed and discussed in the paper.

The second Radiative Heating in Underexplored Bands Campaign (RHUBC-II) was conducted in 2009 by the U.S. Department of Energy Atmospheric Radiation Measurement program to improve water vapor spectroscopy in the far-infrared spectral region. RHUBC-II was located in an extremely dry region of Chile to ensure very low opacities in this spectral region. Spectrally resolved measurements by a far-infrared spectrometer and a submillimeter interferometer from RHUBC-II are compared with line-by-line radiative transfer model calculations. Water vapor amounts and temperatures used in the calculations come from collocated radiosondes, with extensive adjustments to correct for issues due to the campaign's dry conditions and mountainous terrain. A reanalysis is also performed of far-infrared measurements taken at the Atmospheric Radiation Measurement North Slope of Alaska site before and during the first RHUBC campaign. These analyses determine that differences between the measurements and model calculations using existing spectroscopic parameters are significant in the far-infrared and submillimeter regions, leading to the derivation of improved water vapor continuum absorption coefficients and air-broadened widths of 74 water vapor lines. The foreign continuum is increased by more than 50% in part of the far-infrared and the widths of more than 20 lines are changed by more than 10%. The uncertainty in the foreign continuum coefficients is estimated as greater than 20% in some spectral regions, primarily a consequence of the uncertainty in the specification of water vapor. The improved far-infrared spectroscopic parameters have a notable impact on calculated spectral radiances and a modest impact on broadband radiative fluxes and heating rates.

Statistics on the occurrence of clear skies, ice and mixed-phase clouds over the Concordia station, in the Antarctic Plateau, are provided for multiple time scales and analysed in relation to simultaneous meteorological parameters measured at the surface. Results are obtained by applying a machine learning cloud identification and classification code (named CIC) to 4 years of measurements between 2012–2105 of down-welling high spectral resolution radiances, measured by the Radiation Explorer in the Far Infrared-Prototype for Applications and Development (REFIR-PAD) spectroradiometer. The CIC algorithm is optimized for Antarctic sky conditions (clear sky, ice clouds, and mixed-phase clouds) and results in a total hit rate of almost 0.98, where 1.0 is a perfect score. Scene truth is provided by LiDAR measurements that are concurrent with REFIR-PAD. The CIC approach demonstrates the key role of far infrared spectral measurements for clear/cloud discrimination and for cloud phase classification. Mean annual occurrences are 72.3 %, 24.9 % and 2.7 % for clear sky, ice and mixed-phase clouds respectively, with an inter-annual variability of a few percent. The seasonal occurrence of clear sky shows a minimum in winter (66.8 %) and maxima (75–76 %) during intermediate seasons. In winter the mean surface temperature is about 9 °C colder in clear conditions than when ice clouds are present. Mixed-phase clouds are observed only in the warm season; in summer they amount to more than one third of total observed clouds. Their occurrence is correlated with warmer surface temperatures. In the austral summer, the mean surface air temperature is about 5 °C warmer when clouds are present than in clear sky conditions. This difference is larger during the night than in daylight hours, likely due to increased solar warming. A comparison of monthly mean results with cloud occurrence/fraction derived from gridded (Level-3) satellite products, from both passive and active sensors, emphasizes the difficulty of adequately inferring cloud/clear-sky properties in the Antarctic region and highlights the ability of the CIC/REFIR-PAD synergy to identify multiple cloud conditions and study their variability at different time scales.

The forthcoming Earth Explorer 9 - FORUM (Far Infrared Outgoing Radiation Understanding and Monitoring) mission will provide global sounding of the Earth's thermal emission to space in the Far-Infrared (FIR, from 100 to 600 cm-1). The on-board Fourier Transform Spectrometer (FTS) will deliver a wide set of TOA radiances during the mission lifetime that will represent a benchmark for future climate missions. Atmospheric water vapour plays a key role in modulating the OLR in the FIR hence the acquired spectra will also enable future improvements of FIR water vapour spectroscopy. The current uncertainty on spectroscopic data and continuum models is one of the main sources of error on the estimation of the atmospheric radiative properties in the FIR, especially in clear sky conditions where the strong absorption provided by the water vapour rotational band is almost totally responsible for the atmospheric extinction. In preparation for the FORUM-EE9 mission launch in 2026, the analysis of the consistency between observations from prototypes of the FORUM FTS with similar spectral resolution and noise performance and RTM simulations can be a first step toward an improvement of the water vapour spectroscopic data. Here we exploit the balloon-borne measurements above Teresina (Brazil) from the Radiation Explorer in the Far InfraRed - Prototype for Applications and Development (REFIR-PAD) and aircraft observations above the North Sea from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS). These measurements are used to assess the performances of different spectroscopic databases (HITRAN, AER, GEISA), coupled to different water vapor continuum parametrizations. The performances are assessed through a consistency analysis between measurements and simulations aimed to inform which best simulates the observed FIR spectra. The work will also demonstrate the capability of FORUM-like instruments to give a reliable contribution to spectroscopic improvements in the FIR.

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.

The longwave downwelling spectral radiance measurements performed by means of the Far-Infrared Radiation Mobile Observation System (FIRMOS) spectrometer at the summit of the Mt. Zugspitze (German Alps) in the Winter 2018–2019, allowed to retrieve the optical and micro-physical properties of ice, mixed and water clouds, showing a good agreement of the statistical relationship between the ice water path and the ice optical depth with the ones from previous works. In this paper the optical depths retrieved from FIRMOS are initially compared with selected cases calculated from backscattering Light Detection And Ranging (LiDAR) data by using a transmittance method. Then, in order to compare the whole FIRMOS dataset, the power-law relationship between backscattering and extinction is used to apply the Klett method and automatize the routine. Minimizing the root mean square differences, the exponent of the relationship, the so called backscatter-extinction coefficient ratio, is assessed to be 0.85 with a variabiliy in the range 0.60–1.10 for ice clouds and 0.50 with a variability within 0.30–0.70 for mixed and water clouds.

Measurements of the spectrum of the atmospheric emission in the far-infrared (FIR) range, between 100 and 667 cm(-1) (100-15 mu m) are scarce because of the detection complexity and of the strong absorption of air at ground level preventing the sounding of the FIR from low altitude. Consequently, FIR measurements need to be made from high-altitude sites or on board airborne platforms or satellites. This paper describes the dataset of FIR spectral radiances of the atmosphere and snow surface emission measured in the 100-1000 cm(-1) range by the Far-Infrared Radiation Mobile Observation System (FIRMOS) instrument during a 2-month campaign carried out from the ground at about 3000m of altitude on the top of Mt. Zugspitze in the German Alps in 2018-2019. This campaign is part of the preparatory activity of a new space FIR mission, named Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM), which is under development by the European Space Agency (ESA). The dataset acquired during the campaign also includes all the additional measurements needed to provide a full characterisation of the observed atmospheric state and the local atmospheric and surface conditions. It includes co-located spectral measurements in the infrared range from 400 to 1800 cm(-1); lidar backscatter profiles; radio soundings of temperature, humidity and aerosol backscatter profiles; local weather parameters; and snow/ice microphysical properties. These measurements provide a unique dataset that can be used to perform radiative closure experiments to improve modelling parameters in the FIR that are not well-characterised, such as water vapour spectroscopy, scattering properties of cirrus clouds, and the FIR emissivity of the surface covered by snow. The consolidated dataset is freely available via the ESA campaign dataset website at https://doi.org/10.5270/ESA-38034ee (Palchetti et al., 2020a)., Earth System Science Data, 13 (9), ISSN:1866-3516, ISSN:1866-3508

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 has been selected in September 2019 as the 9th Earth Explorer mission of the European Space Agency. The mission aims to measure the Earth’s Top-Of- Atmosphere (TOA) emission spectrum in the spectral region from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). This will fill the current observational gap from space in the far-infrared region (FIR) from 100 to 667 cm-¹ (i.e. from 15 to 100 μm). FORUM measurements will improve the understanding of the climate system by providing, for the first time with high resolution, the spectral features of the far-infrared emission of the Earth with a focus on the contribution to the radiation budget of the continuum absorption of the water vapour rotational bands, on cirrus cloud properties, and on ice/snow surface emissivity. The FORUM mission requires a payload able to spectrally-resolve the Earth's outgoing longwave radiation across the Far-InfraRed (FIR) spectral range with high absolute radiometric accuracy. Characterisation of the surface, atmospheric and cloud/surface heterogeneity in the observed field-of-view is also required to help interpret the measured spectral radiance. These needs dictate the use of two instruments: a spectrometer and an infrared imager. The concepts of both instruments, thoroughly studied in phase A preparatory activities by two independent industrial consortia, are presented in this paper.

The Radiation Explorer in the Far Infrared – Prototype for Applications and Development (REFIR-PAD) is a Fourier transform spectroradiometer that has been designed to operate from both stratospheric balloon platforms and the ground. It has been successfully deployed in a stratospheric balloon flight and several ground-based campaigns from high-altitude sites, including the current installation at the Italian–French Concordia Antarctic station. The instrument is capable of operating autonomously with only a limited need of remote control and monitoring and provides a multiyear dataset of spectrally resolved atmospheric downwelling radiances, measured in the 100–1500 cm−1 spectral range with 0.4 cm−1 resolution and a radiometric uncertainty of better than 0.85 mW(m2srcm-1)-1.

The possibility separating the contributions of the atmospheric state and ice clouds by using spectral infrared measurements is a fundamental step to quantifying the cloud effect in climate models. A simultaneous retrieval of cloud and atmospheric parameters from infrared wideband spectra will allow the disentanglement of the spectral interference between these variables. In this paper, we describe the development of a code for the simultaneous retrieval of atmospheric state and ice cloud parameters, and its application to the analysis of the spectral measurements acquired by the Radiation Explorer in the Far Infrared – Prototype for Applications and Development (REFIR-PAD) spectroradiometer, which has been in operation at Concordia Station on the Antarctic Plateau since 2012. The code performs the retrieval with a computational time that is comparable with the instrument acquisition time. Water vapour and temperature profiles and the cloud optical and microphysical properties, such as the generalised effective diameter and the ice water path, are retrieved by exploiting the 230–980 cm−1 spectral band. To simulate atmospheric radiative transfer, the Line-By-Line Radiative Transfer Model (LBLRTM) has been integrated with a specifically developed subroutine based on the δ-Eddington two-stream approximation, whereas the single-scattering properties of cirrus clouds have been derived from a database for hexagonal column habits. In order to detect ice clouds, a backscattering and depolarisation lidar, co-located with REFIR-PAD has been used, allowing us to infer the position and the cloud thickness to be used in the retrieval. A climatology of the vertical profiles of water vapour and temperature has been performed by using the daily radiosounding available at the station at 12:00 UTC. The climatology has been used to build an a priori profile correlation to constrain the fitting procedure. An optimal estimation method with the Levenberg–Marquardt approach has been used to perform the retrieval. In most cases, the retrieved humidity and temperature profiles show a good agreement with the radiosoundings, demonstrating that the simultaneous retrieval of the atmospheric state is not biased by the presence of cirrus clouds. Finally, the retrieved cloud parameters allow us to study the relationships between cloud temperature and optical depth and between effective particle diameter and ice water content. These relationships are similar to the statistical correlations measured on the Antarctic coast at Dumont d'Urville and in the Arctic region.

The outgoing longwave radiation (OLR) emitted to space is a fundamental component of the Earth’s energy budget. There are numerous, entangled physical processes that contribute to OLR and that are responsible for driving, and responding to, climate change. Spectrally resolved observations can disentangle these processes, but technical limitations have precluded accurate space-based spectral measurements covering the far infrared (FIR) from 100 to 667 cm−1 (wavelengths between 15 and 100 µm). The Earth’s FIR spectrum is thus essentially unmeasured even though at least half of the OLR arises from this spectral range. The region is strongly influenced by upper-tropospheric–lower-stratospheric water vapor, temperature lapse rate, ice cloud distribution, and microphysics, all critical parameters in the climate system that are highly variable and still poorly observed and understood. To cover this uncharted territory in Earth observations, the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has recently been selected as ESA’s ninth Earth Explorer mission for launch in 2026. The primary goal of FORUM is to measure, with high absolute accuracy, the FIR component of the spectrally resolved OLR for the first time with high spectral resolution and radiometric accuracy. The mission will provide a benchmark dataset of global observations which will significantly enhance our understanding of key forcing and feedback processes of the Earth’s atmosphere to enable more stringent evaluation of climate models. This paper describes the motivation for the mission, highlighting the scientific advances that are expected from the new measurements.

FIRMOS (Far-Infrared Radiation Mobile Observation System) is a Fourier transform spectrometer covering the range from 100 to 1000 cm-1, developed to support the preparation of the satellite mission FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) planned in 2025/2026 (Earth Explorer 9 - ESA). FIRMOS was deployed at the Alpine observatory of Mt. Zugspitze, in the South of Germany, at about 3000 m a.s.l. for 2 months (December 2018 and January-February 2019). In January-February 2019, FIRMOS acquired about 600 measurements of the downwelling spectral radiance in clear sky condition. In this presentation, we present results of the retrieval of the water vapour and temperature profiles obtained from Level 2 analysis of the FIRMOS measurements in clear sky condition. Dedicated and coincident measurements allowed us to perform a comparison with integrated water vapour obtained from E-AERI (Extended-range Atmospheric Emitted Radiance Interferometer) at Zugspitze summit station and with radio-sounding launched at Garmisch-Partenkirchen. Comparisons with further independent information are also shown. Acknowledgments. The authors gratefully acknowledge the funding support by the European Space Agency with the FIRMOS project (ESA-ESTEC Contract No. 4000123691/18/NL/LF) and the Italian Space Agency with the research projects SCIEF (Italian acronym of Development of the National Competences for the FORUM experiment - ASI contract No. 2016-010-U.0)

This work aims at determining the best performing mid and far-infrared (MIR and FIR) joint spectral interval to identify and classify clouds in the Antarctic region by mean of a machine learning algorithm.About 1700 spectral-resolved radiances, collected during 2013 by the ground based Radiation Explorer in the Far InfraRed-Prototype for Applications and Development, REFIR-PAD (Palchetti et al., 2015) at Dome C, Antarctic Plateau, are selected in coincidence with the co-located with backscatter and depolarization profiles derived from a tropospheric lidar system (Ricaud et al., 2017) to pre-classify clear sky, ice clouds, or mixed phase clouds.A machine learning cloud identification and classification algorithm named CIC (Maestri et al., 2019), trained with a pre-selected set of REFIR-PAD spectra, is applied to this dataset by assuming that no other information than the spectrum itself is known.The CIC algorithm is applied by considering different spectral intervals, in order to maximize the classification results for each class (clear sky, ice clouds, mixed phase clouds). A CIC "threat score" is defined as the classification true positives divided by the sum of true positives, false positives, and false negatives. The maximization of the threat score is used to assess the algorithm performances that span from 58% to 96% in accordance with the selected interval. The best performing spectral range is the 380-1000 cm-1. The result, besides suggesting the importance of a proper algorithm calibration in accordance with the used sensor, highlights the fundamental role of the FIR part of the spectrum.The calibrated CIC algorithm is then applied to a larger REFIR-PAD dataset of about 90000 spectra collected from 2012 to 2015. Some results of the full dataset cloud classification are also presented.The present work contributes to the preparatory studies for the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission that has recently been selected as ESA’s 9th Earth Explorer mission, scheduled for launch in 2026. References:Maestri, T., Cossich, W., and Sbrolli, I., 2019: Cloud identification and classification from high spectral resolution data in the far infrared and mid-infrared, Atmos. Meas. Tech., 12, pp. 3521 - 3540Palchetti, L., Bianchini, G., Di Natale, G., and Del Guasta, M., 2015: Far infrared radiative properties of water vapor and clouds in Antarctica. Bull. Amer. Meteor. Soc., 96, 1505–1518, doi: http://dx.doi.org/10.1175/BAMS-D-13-00286.1.Ricaud, P., Bazile, E., del Guasta, M., Lanconelli, C., Grigioni, P., and Mahjoub, A., 2017: Genesis of diamond dust, ice fog and thick cloud episodes observed and modelled above Dome C, Antarctica, Atmos. Chem. Phys., 17, 5221–5237, https://doi.org/10.5194/acp-17-5221-2017.

In this paper, we present the retrieval of midlatitude ice, liquid water and mixed-phase cloud parameters from the longwave downwelling spectral radiance measured with the Far-Infrared Radiation Mobile Observation System (FIRMOS) spectrometer. FIRMOS, developed within an ESA Project for the preparation of the FORUM mission, was installed for 2 months (December 2018 and January-February 2019) at the Alpine observatory of Mt. Zugspitze, in the South of Germany, at about 3000 m a.s.l. The contemporary availability of a backscattering Lidar, located at the Schneefernerhaus Station, 300 m below the Zugspitze summit, allowed to constrain the cloud position in the radiative transfer model and to retrieve optical depths, phase and cloud microphysics with improved accuracy. A good agreement is found between the optical depth values obtained from the Lidar and those from radiance measurements.

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.

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.

In this study we introduce a new strategy for the measurement of CO2 distribution in the stratosphere. The proposed experiment is based on an orbiting limb sounder that measures the atmospheric emission within both the thermal infrared (TIR) and far-infrared (FIR) regions. The idea is to exploit the contribution of the pure rotational transitions of molecular oxygen in the FIR to determine the atmospheric fields of temperature and pressure that are necessary to retrieve the distribution of CO2 from its rovibrational transitions in the TIR. The instrument envisaged to test the new strategy is a Fourier transform spectrometer with two output ports hosting a FIR detector devoted to measuring the O2 transitions and a TIR detector devoted to measure the CO2 transitions. Instrumental and observational parameters of the proposed experiment have been defined by exploiting the heritage of both previous studies and operational limb sounders. The performance of the experiment has been assessed with two-dimensional (2-D) retrievals on simulated observations along a full orbit. For this purpose, optimal spectral intervals have been defined using a validated selection algorithm. Both precision and spatial resolution of the obtained CO2 distributions have been taken into account in the results–evaluation process. We show that the O2 spectral features significantly contribute to the performance of CO2 retrievals and that the proposed experiment can determine 2-D distributions of the CO2 volume mixing ratio with precisions of the order of 1 ppmv in the 10–50 km altitude range. The error budget, estimated for the test case of an ideal instrument and neglecting the spectroscopic errors, indicates that, in the 10–50 km altitude range, the total error of the CO2 fields is set by the random component. This is also the case at higher altitudes, provided the retrieval system is able to model the non-local thermal equilibrium conditions of the atmosphere. The best performance is obtained at altitudes between 20 and 50 km, where the vertical resolution ranges from 3 to 5 km, and the horizontal resolution is of the order of 300–350 km depending on latitude.

The thermal emission of cirrus clouds, spectrally-resolved in the 100–1400 cm − 1 range (100–7.1 μm), has been modeled and compared with measurements performed during two field campaigns from the ground-based site of Testa Grigia on the Italian Alps at 3480 m of altitude. The analysis of cirrus microphysics, through spectral fitting, shows the importance of using also the far infrared portion of the emitted spectrum at wavenumbers below the 667 cm − 1 carbon dioxide absorption band, where only a few measurements exist because of the high opacity of the atmosphere caused by the strong water vapor absorption. The resulted distribution of the fitted cloud parameters is in good agreement with the typical statistical distribution of the midlatitude cirrus cloud parameters.

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.

One year (2013) of high spectral resolution measurements of downwelling radiance in the 100-1,400 cm(-1) range, taken by the Fourier Transform Spectrometer REFIR-PAD at the research station Concordia (Antarctic Plateau), is analyzed. Optically thin ice clouds are identified by means of a new identification/classification tool based on a Support Vector Machine algorithm. The use of transparent microwindow channels in the Far InfraRed (FIR) spectral region (100-667 cm(-1)) is shown to be of great importance in the identification and classification of cloud type. In particular, the channels between 380 and 575 cm(-1) are key channels for the clear/cloud and phase identification due to their sensitivity to cloud properties; in addition, FIR channels down to 238 cm(-1) are exploited for the selection of precipitating or nonprecipitating cases because of their sensitivity also to water vapor content. A subset of 26 cases of nonprecipitating ice clouds is selected based on the presence of colocated LIDAR and radiosonde data. REFIR-PAD channel in the 800-1,000 cm(-1) are used to derived optical and microphysical properties for four different assumptions concerning the crystal habits. Results, showing a correlation between cloud base temperature, optical depth, and particle size distribution effective dimensions, are compared with what found in literature. Based on the retrievals, forward simulations are also run over the whole sensor spectral interval, and results are compared to data. The simulation-data residuals in the FIR are evaluated for selected "window" channels and analyzed in relation to crystal's habit assumption, cloud retrieved features, and atmospheric water vapor content.

Since December, 2011 an integrated observation system based on the REFIR-PAD Fourier transform spectrora- diometer has been established at the station Concordia, on the East Antarctic plateau (75 o 06'S, 123 o 23'E, 3233 m a. s. l.). The REFIR-PAD instrument provides spectrally-resolved measurements of the downwelling atmospheric emitted radiance in the 100-1500 cm -1 range, thus including the far-infrared region; together with a set of auxiliary sensors connected to a single data acquisition and storage system these measurements provide a valuable tool for the characterization of the Antarctic troposphere. The observation system is operating in continuous acquisition mode since its installation, thus providing an unique, more than 7-year long, time series of atmospheric radiances and ancillary data. Polar regions are the main radiative sink of the Earth system, thus the monitoring of their atmosphere has a high rel- evance for climate studies. Broad band, spectrally resolved radiance measurements as those performed by REFIR- PAD fulfill a twofold task, providing a characterization of the atmospheric radiation including the identification of its main contributing components, but also allowing for the retrieval of the atmospheres structure and composition through the analysis of the measured spectra. This kind of analysis is particularly interesting in the polar context, since it includes the far-infrared region of the spectrum in which, due to the low temperatures involved, a great part of the radiative exchange takes place. A description of the currently operating observation system will be presented, together with some preliminary results from the analysis of the measurements performed in the 2012-2018 period.

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.

HAPS: fixed WINGS or LTA : limits and potentiality The presentation will discuss the limit and potentiality of the typology of two different HAPs. A fixed wings and a LTA platform. The two performances will be compared in the specific case of potential use of two proposed HAP in the framework of two different systems for on-going project and studies for deployment from space of innovative observation systems and concepts aimed at monitoring of the Earth's atmosphere. The first one is the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, candidate mission for the Earth Explorer 9 programme, currently under competitive feasibility study. The second is the NDSA (Normalized Differential Spectral Attenuation) measurement of water vapour in the troposphere, an innovative technique for active sounding of atmospheric water vapour between two co-rotating (or counter-rotating) LEO satellites. Possible scenarios will be described in some detail of perspectives to deploy onboard the HAP an airborne version of the FORUM instrument and/or a demonstrator of the NDSA technique applied to water vapour measurements from a flying platform.

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.

Far-infrared (FIR: 100cm-1

Water vapor and clouds are among the most important greenhouse components whose radiative features cover all the broad spectral range of the thermal emission of the atmosphere. Typically more than 40% of the total thermal emission of Earth occurs in the far-infrared (FIR) spectral region from 100 to 667 cm−1 (wavelengths from 100 to 15 µm). Nevertheless, this spectral region has not ever been fully covered down to 100 cm−1 by space missions, and only a few ground-based experiments exist because of the difficulty of performing measurements from high altitude and very dry locations where the atmosphere is sufficiently transparent to observe the FIR emission features. To cover this lack of observations, the Italian experiment “Radiative Properties of Water Vapor and Clouds in Antarctica” has collected a 2-yr dataset of spectral measurements of the radiance emitted by the atmosphere and by clouds, such as cirrus and polar stratospheric clouds, from 100 to 1,400 cm−1 (100–7 µm of wavelength), including the underexplored FIR region, along with polarization-sensitive lidar observations, daily radiosondes, and other ancillary information to characterize the atmosphere above the site. Measurements have been performed almost continuously with a duty cycle of 6 out of 9 h, from the Italian–French base of Concordia at Dome C over the Antarctic Plateau at 3,230 m MSL, in all-sky conditions since 2012. Because of the uniqueness of the observations, this dataset will be extremely valuable for evaluating the accuracy of atmospheric absorption models (both gas and clouds) in the underexplored FIR and to detect possible daily, seasonal, and annual climate signatures.

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.