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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.

In the framework of the ACCLIP project (Asian summer monsoon Chemical and CLimate Impacts project), a measurement campaign was conducted during summer 2022 in the Western Pacific region, to investigate the impact of the Asian Summer Monsoon (ASM) on the composition of the upper troposphere and lower stratosphere (UTLS). Fifteen research flights were carried out by the NASA WB-57 stratospheric aircraft and 14 by the NCAR/NSF GV, with base in Osan (South Korea), covering a large region on the eastern edge of the ASM anticyclone.We report on the Carbon Monoxide (CO) measurements performed by three different mid-infrared absorption spectrometers (COLD2, COMA and ACOS) installed onboard the WB-57 and by two different infrared absorption spectrometers (Aerodyne-CO and Picarro G2401) installed on the GV. Positive CO anomalies, never measured before in the UTLS outside direct biomass burning plumes, were collected by all sensors, showing a very good agreement. During the flight of the 19th of August, CO mixing ratio values higher than 250 ppb were registered at altitude around 14-15 km.A comparison with the CO observations measured by the instrument COLD2 during the StratoClim (Stratospheric and upper tropospheric processes for better Climate predictions) campaign, conducted in summer 2017 from Kathmandu (Nepal), will be presented. Particular attention will be paid to the CO difference observed in the UTLS, by sampling the anticyclone directly from the Tibetan Plateau during StratoClim campaign or from the Western Pacific during ACCLIP.

The summertime Asian Monsoon (AM) is the single most important contributor to water vapor in the UTLS and overworld stratosphere. Much of that water comes from sublimating ice, but the life cycle of the condensate lofted by overshooting convection is not well understood. We report here on insights into that life cycle derived from the first in-situ measurements of water vapor isotopic composition over the Asian Monsoon. The Chicago Water Isotope Spectrometer (ChiWIS) flew on high-altitude aircraft in the monsoon center during the StratoClim (2017) campaign out of Nepal, and in monsoon outflow during ACCLIP (2022) out of South Korea. Both campaigns sampled a broad range of convective and post-convective conditions, letting us trace how convective ice sublimates, reforms, and leaves behind characteristic isotopic signatures. We use the Bin Resolved Isotopic Microphysical Model (BRIMM), along with TRACZILLA backtrajectories and convective interactions derived from cloud-top products, to follow the evolving isotopic composition along flight paths in both campaigns. Results support the wide diversity of isotopic enhancements seen in both campaigns and show how temperature cycles downstream of convective events progressively modify environmental isotopic compositions.

The 2022 ACCLIP (Asian summer monsoon Chemical and CLimate Impact Project) high-altitude aircraft campaign has provided a sampling of the diversity of processes that affect moisture transport in the upper troposphere / lower stratosphere (UT/LS). We report here on ACCLIP observations of water vapor isotopologues, which trace the origin and microphysical history of water vapor. Measurements with the Chicago Water Isotope Instrument (ChiWIS) show isotopic variations in the UT/LS that correlate with airmass history, and 100-150 ‰ variation even at the same water content. ACCLIP flights out of Osan, South Korea sampled monsoon anticyclone outflow with CO values over 200 ppb, recent local convection, extensive in-situ cirrus, and an overflight of tropical cyclone Hinnanmor showing strong isotopic depletion. Flights out of Houston, TX sampled week-old remnants of sublimated ice from deep convection, producing enriched vapor, and possible mixing of convective overshoots with stratospheric air before sinking. We show through case studies from both Asia and North America that isotopologues provide a sensitive diagnostic of ice sublimation, and demonstrate how different meteorological contexts produce distinct isotopic signatures.

A series of in-situ Carbon Monoxide (CO) observations were recently performed in the Western Pacific region, during summer 2022, in the framework of the ACCLIP project (Asian summer monsoon Chemical and CLimate Impacts Project). During the ACCLIP measurements campaign, located in Osan (South Korea), two different research aircraft were employed with a set of sensors installed onboard. The NASA WB-57 aircraft carried out 15 research flights (reaching a maximum altitude of about 19 km), and the NSF/NCAR Gulfstream (GV) aircraft carried out 14 research flights (reaching a maximum altitude of about 15 km), covering a large region near Korea and Japan.We report on the inter-comparison between five different instruments for in-situ CO mixing ratio measurements: three installed onboard WB-57 (ACOS, COLD2 and COMA), and two installed onboard GV (Aerodyne-CO and Picarro G2401-m). COLD2 (Carbon Oxide Laser Detector 2) [1] and Aerodyne-CO [2] are mid-infrared Quantum Cascade Laser spectrometers, based on direct absorption in combination with a multipass cell. ACOS (Carbonyl Sulfide Analyzer) [3] and COMA (Carbon mOnoxide Measurement from Ames) [4] are mid-infrared absorption spectrometers based on Off-Axis ICOS (Integrated Cavity Output Spectroscopy) technology. The Picarro sensor is a cavity ring down absorption spectrometer [5].The in-flight CO mixing ratio values measured by the five spectrometers will be compared, with particular attention to both the accuracy of each instrument and the adopted or not-adopted calibration procedures, as, in principle, for many measurement environments the two sensors based on direct absorption do not need in-flight calibration. Laboratory measurements of common primary and secondary calibration standards made by the five CO measurement groups will be presented to increase confidence in method accuracy. [1] Viciani S., Montori A., Chiarugi A., and D’Amato F.: "A Portable Quantum Cascade Laser Spectrometer for Atmospheric Measurements of Carbon Monoxide", Sensors, 18, 2380 -1-18 (2018).[2] https://www.eol.ucar.edu/instruments/carbon-monoxide-co-and-nitrous-oxide-n%E2%82%82o-qcl-instrument[3] https://ams.confex.com/ams/103ANNUAL/meetingapp.cgi/Paper/421824[4] https://espo.nasa.gov/acclip/instrument/COMA[5] https://www.eol.ucar.edu/instruments/airborne-carbon-dioxide-co2-methane-ch4-carbon-monoxide-co-and-water-vapor-h2o

The paper presents the first retrievals of clean snow properties using spaceborne hyperspectral The Environmental Mapping and Analysis Program (EnMAP) observations. The location close to the Dome C in Antarctica has been selected. At this location the atmospheric effects except molecular light scattering and absorption are weak and the simplified atmospheric correction scheme can be applied. The ice grain size, snow specific surface area and snow spectral and broadband albedos have been retrieved using single view EnMAP measurements. In addition, we propose the technique to retrieve trace gas concentrations (e.g., water vapor, ozone) from EnMAP observations over the snow surfaces. Close correspondence of satellite and ground measured parameters has been found.

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.

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.

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.

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

The 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.

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.

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 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.

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.

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.

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.

Cirrus clouds play a key role in the Earth's radiation budget but there is a large uncertainty about their infrared ra- diative impact due to the high variability in the size/shape distribution of ice particles. Their radiative effect is very strong in the atmospheric window between 820-960 cm -1 (8-12 ?m) but the contribution in the far infrared (FIR) of the spectrum, below 667 cm -1 (> 15 ?m), is also very important. In fact about 50% of the entire thermal flux is emitted in the FIR spectral range and here the sensitivity to the ice particle properties, such as crystal habits, is very large. Despite this, only few measurements exist in the FIR that could be used to improve the characterisation of cirrus radiative effect. The present work describes remote sensing data concerning high latitudes ice clouds and the derivation of their optical and micro-physical properties, such as the crystals effective diameter, the ice water path and optical depth. Measurements are performed by means of a Fourier transform spectroradiometer, named Radiation Explorer Far Infrared - Prototype for Applications and Development (REFIR-PAD), operating in the FIR spectral region. This instrument has been successfully used during many field campaigns and it is currently installed at Concordia base at Dome-C (Antarctica, 3230 m a.s.l.), where has been operating continuously in ground-based zenith-looking ob- servation geometry since 2012. This campaign has allowed to collect a wide database of the downwelling spectral radiance, also in presence of cirrus clouds, together with other supporting measurements, such as lidar and ther- modynamic vertical profiles, for independent characterisation of the atmospheric state. The retrieval of the cirrus clouds parameters has been performed by two independent methodologies: the physical Radiative Transfer - Retrieval (RT-RET) and the Simultaneous Atmospheric and Clouds Retrieval (SACR) code which is based on optimal estimation. This last code, developed for this purpose, allows to perform the retrieval of the atmospheric variables, such as the vertical profiles of water vapour and temperature, and the cirrus clouds properties by using the single scattering coefficients of different ice crystal habits, provided by specific databases. The analysis of the cirrus clouds properties, performed by using the ground-based measurements of REFIR-PAD, will be of support to the Far-infrared-Outgoing-Radiation Understanding and Monitoring (FORUM) mission, re- cently selected by ESA as candidate for the EE-9 missions. FORUM will perform for the first time the spectral measurement of the FIR outgoing radiation and, by flying in tandem with IASI-NG which measures the mid-IR component, will open up a new window for understanding and quantifying the radiative processes involving water vapour, clouds and other greenhouse gases.

This paper analyses high spectral resolution downwelling radiance measurements in the far infrared in the presence of cirrus clouds taken by the REFIR-PAD interferometer, deployed at 3500 m above the sea level at the Testa Grigia station (Italy), during the Earth COoling by WAter vapouR emission (ECOWAR) campaign. Atmospheric state and cloud geometry are characterised by the co-located millimeter-wave spectrometer GBMS and by radiosonde profile data, an interferometer (I-BEST) and a Raman lidar system deployed at a nearby location (Cervinia). Cloud optical depth and effective diameter are retrieved from REFIR-PAD data using a limited number of channels in the 820–960 cm−1 interval. The retrieved cloud parameters are the input data for simulations covering the 250–1100 cm−1 band in order to test our ability to reproduce the REFIR-PAD spectra in the presence of ice clouds. Inverse and forward simulations are based on the same radiative transfer code. A priori information concerning cloud ice vertical distribution is used to better constrain the simulation scheme and an analysis of the degree of approximation of the phase function within the radiative transfer codes is performed to define the accuracy of computations. Simulation-data residuals over the REFIR-PAD spectral interval show an excellent agreement in the window region, but values are larger than total measurement uncertainties in the far infrared. Possible causes are investigated. It is shown that the uncertainties related to the water vapour and temperature profiles are of the same order as the sensitivity to the a priori assumption on particle habits for an up-looking configuration. In case of a down-looking configuration, errors due to possible incorrect description of the water vapour profile would be drastically reduced.

The spectral features of the downward longwave radiation have been characterized by exploiting spectral measurements, acquired over the Antarctic Plateau at Dome C at 3230 m, by the Radiation Explorer in the Far InfraRed - Prototype for Applications and Development (REFIR-PAD) spectroradiometer. REFIR-PAD is operative at the base with continuous operations over 24 hours since 2012 and measures the spectra from 100 to 1400 cm−1 with 0.4 cm−1 of resolution, covering the under-explored far-IR for the first time with systematic measurements. REFIR-PAD spectral measurements have been used to calculate the atmospheric longwave flux to be compared with standard measurements performed by BSRN, for validation and error estimates. The agreement is within the error estimate of 4.5 W m−2 for REFIR-PAD and 10 W m−2 for BRSN measurements. Finally, seasonal variabilities of the longwave flux measured in different spectral regions have been studied by exploiting 1-year dataset of measurements to show possible correlation...

La radiazione termica emessa dalla Terra verso lo spazio copre un ampio intervallo di frequenze che vanno dal medio al lontano InfraRosso (FIR). Le misure dallo spazio tipicamente coprono il medio IR lasciando il FIR (100-667cm -1 ) sostanzialmente inesplorato. Di recente è stata proposta per le EarthExplorer-9 di ESA una missione con leadearship italiana, chiamata FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring), che ha l'obiettivo di coprire tutto l'intervallo spettrale, misurando per la prima volta dallo spazio tutto il FIR, fondamentale per la caratterizzazione del bilancio radiativo terrestre ed il miglioramento delle previsioni climatiche e meteorologiche attraverso una determinazione più accurata dei profili verticali di vapore acqueo e temperatura e delle proprietà radiative dei cirri. La misura proposta in FORUM può essere realizzata anche da pallone stratosferico con uno spettrometro FT che copra l'intervallo spettrale 100-1600cm -1 . Le capacità di sviluppare tale strumento sono ben consolidate in Italia, basandosi su lavori iniziati alcuni anni fa con il progetto REFIR e lo sviluppo del prototipo REFIR-PAD, operante sia da pallone che in misure da terra in alta quota (Cile, Alpi, Antartide). La comunità scientifica italiana coinvolta è ampia e presente presso le Università e Centri di ricerca, e lo sviluppo tecnologico è ben supportato dall'industria nazionale, principalmente da Leonardo. Piattaforme da alta quota, come i palloni stratosferici, permetteranno sia di testare eventuali prototipi della missione FORUM, nel caso fosse selezionata, fornendo anche supporto alla validazione, sia di effettuare osservazioni mirate alla risoluzione di problematiche scientifiche ancora aperte nella caratterizzazione del FIR.

The possibility to discriminate the contribution of the atmosphere and ice clouds by using spectral infrared measurements is a fundamental step to quantify the cloud effect in the climate models. The simultaneous retrieval of clouds and atmospheric parameters allows to take into account the possible correlations between the variables. In this paper we describe the development of a routine able to perform the simultaneous retrieval and its application to the analysis of the spectral measurements acquired by the REFIR-PAD (Radiation Explorer Far Infrared – Prototype for Applications and Development) spectroradiometer, operative from Antarctic Plateau since 2012. This routine is able to operate the retrieval with reduced computing time comparable with the REFIR-PAD data acquisition time. The analysis allowed to retrieve the water vapour and temperature profiles and the clouds optical and micro-physical properties, such as the generalised effective diameter (Dge) and the ice water path (IWP), by exploiting the spectral band between 230–980 cm−1. To simulate the radiative transfer, the LBLRTM (Line By Line Radiative Transfer Model) was integrated with a specific code based on the delta-Eddington two-stream approximation, and further the cirrus clouds single scattering properties were derived from a database for hexagonal column habits. In order to identify the ice clouds a back-scattering and depolarisation lidar was available on the site and allowed to infer the position and the cloud thickness used in the simulations. A climatology of the vertical profiles of water vapour and temperature, was performed by using the daily radiosounding available at the base and it was used to build the a priori and initial guess profiles used in the fitting routine. An optimal estimation method with a Levenberg-Marquardt approach was applied to perform the retrieval. The comparison of results with radiosoundings demonstrates that the retrieved atmospheric state is not disturbed by the clouds presence. Finally, the retrieved clouds parameters were compared with the statistical correlation between the cloud temperature (Tc) and the optical depth measured in Antarctica at Dumond D'Urville and the effective diameters with the ice water content (IWC) obtained in the Arctic region finding a general agreement.

The design and characterisation of small and cost-effective reference black-bodysources for radiometric calibration of Fourier transform spectrometers in the midand far infrared is presented. The paper describes the optimisation of the absorbingblack-coating material and the study of two types of cavity geometry, one designedfor collimated beams and one for focused beams. These two reference sources havebeen developed for the Radiation Explorer in the Far InfraRed spectrometer cov-ering the 100-1400 cm −1 spectral range. They are currently used on-board twoprototypes for ground and stratospheric balloon based atmospheric emission mea-surements. An emissivity of better than 0.9999 is obtained for both type of cavitygeometry giving a calibration error, depending only on the temperature gradientinside the blackbody, of less than 0.5 K. Keywords: Fourier transform spectra, Infrared spectroscopy, Blackbody radiation PACS: 33.20.Ea, 07.57.Ty, 44.40.+a 1 IntroductionIn emission spectroscopy, the calibration of the observed radiance requiresthe measurement of a known source of radiation, which typically is givenby the emission of a blackbody (BB) cavity described by the Planck law ofradiation. The design of a well-characterised and reliable BB source can be

A retrieval method for cirrus clouds was applied to the measurements of spectral radiance performed by the REFIR-PAD Fourier spectroradiometer over the Antarctic Plateau at Dome C.

Far-IR spectral measurements of the Earth's longwave emission can be used to improve the characterization of the radiative effects of water vapor, cirrus clouds and surface properties.© 2016 Optical Society of America

Atmospheric emitted radiance, provided by a FT spectroradiometer installed at Concordia Station, Antarctica in the framework of the COMPASS project has been analyzed providing a characterization of the Antarctic troposphere in the 2014-2016 period.

Nitric acid (HNO3) is one of the key products that are operationally retrieved by the European Space Agency (ESA) from the emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard ENVISAT. The product version 4.61/4.62 for the observation period between July 2002 and March 2004 is validated by comparisons with a number of independent observations from ground-based stations, aircraft/balloon campaigns, and satellites. Individual HNO3 profiles of the ESA MIPAS level-2 product show good agreement with those of MIPAS-B and MIPAS-STR (the balloon and aircraft version of MIPAS, respectively), and the balloon-borne infrared spectrometers MkIV and SPIRALE, mostly matching the reference data within the combined instrument error bars. In most cases differences between the correlative measurement pairs are less than 1 ppbv (5–10%) throughout the entire altitude range up to about 38 km (~6 hPa), and below 0.5 ppbv (15–20% or more) above 30 km (~17 hPa). However, differences up to 4 ppbv compared to MkIV have been found at high latitudes in December 2002 in the presence of polar stratospheric clouds. The degree of consistency is further largely affected by the temporal and spatial coincidence, and differences of 2 ppbv may be observed between 22 and 26 km (~50 and 30 hPa) at high latitudes near the vortex boundary, due to large horizontal inhomogeneity of HNO3. Similar features are also observed in the mean differences of the MIPAS ESA HNO3 VMRs with respect to the ground-based FTIR measurements at five stations, aircraft-based SAFIRE-A and ASUR, and the balloon campaign IBEX. The mean relative differences between the MIPAS and FTIR HNO3 partial columns are within ±2%, comparable to the MIPAS systematic error of ~2%. For the vertical profiles, the biases between the MIPAS and FTIR data are generally below 10% in the altitudes of 10 to 30 km. The MIPAS and SAFIRE HNO3 data generally match within their total error bars for the mid and high latitude flights, despite the larger atmospheric inhomogeneities that characterize the measurement scenario at higher latitudes. The MIPAS and ASUR comparison reveals generally good agreements better than 10–13% at 20–34 km. The MIPAS and IBEX measurements agree reasonably well (mean relative differences within ±15%) between 17 and 32 km. Statistical comparisons of the MIPAS profiles correlated with those of Odin/SMR, ILAS-II, and ACE-FTS generally show good consistency. The mean differences averaged over individual latitude bands or all bands are within the combined instrument errors, and generally within 1, 0.5, and 0.3 ppbv between 10 and 40 km (~260 and 4.5 hPa) for Odin/SMR, ILAS-II, and ACE-FTS, respectively. The standard deviations of the differences are between 1 to 2 ppbv. The standard deviations for the satellite comparisons and for almost all other comparisons are generally larger than the estimated measurement uncertainty. This is associated with the temporal and spatial coincidence error and the horizontal smoothing error which are not taken into account in our error budget. Both errors become large when the spatial variability of the target molecule is high.