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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. [ABSTRACT FROM AUTHOR]

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 both from stratospheric balloon platform and from ground. It has been successfully deployed in a stratospheric balloon flight and several ground based campaigns from high altitude sites, including the current installation in the Concordia Italian-French Antarctic station. The instrument is capable to operate autonomously with only a limited need of remote control and monitoring, and is providing a multi-year dataset of spectrally resolved atmospheric downwelling radiances, measured in the 100-1500cm-1 spectral range with 0.4cm-1 resolution and a radiometric uncertainty better than 0.85mW/m2srcm-1. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Selected case studies of precipitating ice clouds at Dome C (Antarctic Plateau) were used to test a new approach for the estimation of ice cloud reflectivity at 24 GHz (12.37 mm wavelength) using ground-based far infrared spectral measurements from the REFIR-PAD Fourier transform spectroradiometer and backscattering/depolarization lidar profiles. The resulting reflectivity was evaluated with the direct reflectivity measurements provided by a co-located micro rain radar (MRR) operating at 24 GHz, that was able to detect falling crystals with large particle size, typically above 600 µ m. 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. These parameters spanned in the wide ranges between 570–2400 µ m and 10-2 – 104 cm -5 , respectively. The retrieved effective sizes and optical depths mostly varied in the ranges 70–250 µ m and 0.1–5, respectively. From these parameters, the theoretical reflectivity at 24 GHz was obtained by integrating the size distribution over different cross sections for various habit crystals provided by databases. From the comparison with the radar reflectivity measurements, we found that the hexagonal column-like habits, the columnar crystal aggregates, and the 5/6 branches bullet rosettes showed the best agreement with the MRR observations. The dispersion coefficient of the crystal particle size distribution was assumed in the range 0–2 according to the temperature dependence found in previous studies. The retrieved values of the intercept and slope were found in good agreement with these studies. The presence of the inferred habits was confirmed by the crystal images taken by the ICE-CAMERA, operating in proximity of REFIR-PAD and the MRR. In particular, the occurrence of hexagonal column-like ice crystals was confirmed by the presence of 22 ∘ solar halos, detected by the HALO-CAMERA. The average crystal lengths obtained from the retrieved size distribution were also compared to those estimated from the ICE-CAMERA images. The agreement between the two results confirmed that the retrieved parameters of the particle size distributions correctly reproduced the observations. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

The accurate measurement of thermal expansion coefficient at low temperatures is fundamental in applications where a high mechanical stability is required over the complete procedure of cooling. Here we report on our measurement of thermal expansion of AISI 420 between 20 and 293K, measured by an interferometric dilatometer. [ABSTRACT FROM AUTHOR]

Optical and microphysical cloud properties are retrieved from measurements acquired in 2013 and 2014 at the Concordia base station in the Antarctic Plateau. Two sensors are used synergistically: a Fourier transform spectroradiometer named REFIR-PAD (Radiation Explorer in Far Infrared-Prototype for Applications and Developments) and a backscattering-depolarization LiDAR. First, in order to identify the cloudy scenes and assess the cloud thermodynamic phase, the REFIR-PAD spectral radiances are ingested by a machine learning algorithm called Cloud Identification and Classification (CIC). For each of the identified cloudy scenes, the nearest (in time) LiDAR backscattering profile is processed by the Polar Threshold (PT) algorithm that allows derivation of the cloud top and bottom heights. Subsequently, using the CIC and PT results as external constraints, the Simultaneous Atmospheric and Clouds Retrieval (SACR) code is applied to the REFIR-PAD spectral radiances. SACR simultaneously retrieves cloud optical depth and effective dimensions and atmospheric vertical profiles of water vapor and temperature. The analysis determines an average effective diameter of 28 μ m with an optical depth of 0.76 for the ice clouds. Water clouds are only detected during the austral Summer, and the retrieved properties provide an average droplet diameter of 9 μ m and average optical depth equal to four. The estimated retrieval error is about 1% for the ice crystal/droplet size and 2% for the cloud optical depth. The sensitivity of the retrieved parameters to the assumed crystal shape is also assessed. New parametrizations of the optical depth and the longwave downwelling forcing for Antarctic ice and water clouds, as a function of the ice/liquid water path, are presented. The longwave downwelling flux, computed from the top of the atmosphere to the surface, ranges between 70 and 220 W/m 2 . The estimated cloud longwave forcing at the surface is (31 ± 7) W/m 2 and (29 ± 6) W/m 2 for ice clouds and (64 ± 12) and (62 ± 11) W/m 2 for water clouds, in 2013 and 2014, respectively. The total average cloud forcing for the two years investigated is (46 ± 9) W/m 2 . [ABSTRACT FROM AUTHOR]

Since December, 2011 an integrated observation system based on the REFIR-PAD Fouriertransform spectroradiometer has been established at the station Concordia, on the EastAntarctic plateau (75∘06’S, 123∘23’E, 3233 m a. s. l.). The REFIR-PAD instrument provides spectrally-resolved measurements of thedownwelling atmospheric emitted radiance in the 100-1500 cm−1 range, thus including thefar-infrared region; together with a set of auxiliary sensors connected to a single dataacquisition and storage system these measurements provide a valuable tool for thecharacterization 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 andancillary data. Polar regions are the main radiative sink of the Earth system, thus the monitoring of theiratmosphere has a high relevance for climate studies. Broad band, spectrally resolved radiancemeasurements as those performed by REFIR-PAD fulfill a twofold task, providing acharacterization of the atmospheric radiation including the identification of its maincontributing components, but also allowing for the retrieval of the atmospheres structure andcomposition through the analysis of the measured spectra. This kind of analysis is particularly interesting in the polar context, since it includes thefar-infrared region of the spectrum in which, due to the low temperatures involved, a greatpart of the radiative exchange takes place. A description of the currently operating observation system will be presented, togetherwith some preliminary results from the analysis of the measurements performed in the2012-2018 period. [ABSTRACT FROM AUTHOR]

The paper presents the first retrievals of clean snow properties using spaceborne hyperspectral observations via the Environmental Mapping and Analysis Program (EnMAP). The location close to the Concordia station at the Dome C Plateau (Antarctica) was selected. At this location, the atmospheric effects (except molecular light scattering and absorption) are weak, and the simplified atmospheric correction scheme could be applied. The ice grain size, snow specific surface area, and snow spectral and broadband albedos were retrieved using single-view EnMAP measurements. In addition, we propose a technique to retrieve trace gas concentrations (e.g., water vapor and ozone) from EnMAP observations over the snow surfaces. A close correspondence of satellite and ground-measured parameters was found. [ABSTRACT FROM AUTHOR]

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 characterization 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. [ABSTRACT FROM AUTHOR]

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 singlescattering 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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

The long-wave downwelling spectral radiance measurements performed by means of the Far-Infrared Radiation Mobile Observation System (FIRMOS) spectrometer at the summit of the Zugspitze (German Alps) in the winter 2018/19 allowed the retrieval of the optical and micro-physical properties of ice and mixed 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 k of the power-law relationship is assessed to be 0.85 with a variability in the range of 0.60–1.10 for ice clouds and 0.50 with a variability within 0.30–0.70 for mixed clouds. [ABSTRACT FROM AUTHOR]

Measurements of the spectrum of the atmospheric emission in the far-infrared (FIR) range, between 100 and 667 cm-1 (100–15 µm) 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 3000 m 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 10.5270/ESA-38034ee. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Measurements of the spectrum of the atmospheric emission in the far-infrared (FIR) range, between 100 and 667 cm-1 (100–15 µm), 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 (FIR-MOS) instrument, during a 2-month campaign carried out from ground at 3000 m of altitude, on the top of Mount Zugspitze in the German Alps, in 2018-2019. This campaign is part of the preparatory activity of a new space FIR mission, named Farinfrared-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; radiosoundings 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 that in the FIR 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). For the review process, the dataset is also available at https://fts.fi.ino.it/forum/firmos/zugspitze-dataset/ [ABSTRACT FROM AUTHOR]

• Introduction of a two-stream δ -Eddington approximation algorithm to simulate the Far InfraRed Earth spectrum. • Study of SACR accuracy by means of the comparison with LBLDIS (LBLRTM+DISORT) code. • Performance of the simultaneous retrieval of atmospheric and cloud parameters by using SACR algorithm. 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. [ABSTRACT FROM AUTHOR]

Ice and mixed phase clouds play a key role in the Earth's radiation budget but there is a large uncertainty about their infrared radiative impact due to the high variability in the size/shape distribution of ice particles. Their radiative effect is very strong in the mid and far infrared (FIR) spectrum, between 200-1000 cm−1 (10-50 μm). In particular the FIR part of the spectrum (under 600 cm-1) shows a strong sensitivity to the ice particle properties, such as crystal habits, and allows the discrimination of the clouds phase, since in this spectral region the refractive index of ice is very different from the water ones. Despite this, only few measurements exist in the FIR that could be used to improve the characterisation of polar clouds radiative effect.The present work describes the synergy of different instruments deployed at Concordia base, Dome C (Antarctica, 3230 m a.s.l.), to study the optical and micro-physical properties of the ice and mixed phase clouds. Currently at Italian-French base Concordia, there are different instruments devoted to the measurements of the Antarctic clouds properties, such as the Fourier transform spectroradiometer REFIR-PAD (Radiation Explorer Far Infrared – Prototype for Applications and Development), operating in the FIR spectral region, a backscattering/depolarization lidar, which allows to monitor the passage of clouds, an ice camera for the estimate of the size distribution of ice particles and, finally, a micro rain radar (MRR) operating at 24 GHz, installed during this last summer campaign in order to estimate the Ice Water Content (IWC) of the clouds. REFIR-PAD spectroradiometer is operating in ground-base zenith-looking observation geometry in continuous mode at Dome-C since December 2011, allowing to collect a very large database of the spectral radiances of the Antarctic atmosphere both in clear and cloudy sky conditions. The retrieval of the clouds parameters is performed by using the spectral measurements of REFIR-PAD and the Simultaneous Atmospheric and Clouds Retrieval (SACR) code, which is based on the optimal estimation approach. This code allows to perform the retrieval of the atmospheric variables, such as the vertical profiles of water vapour and temperature, and the clouds properties simultaneously, by using the single scattering coefficients of different ice crystal habits, provided by specific databases.The SACR algorithm allows to retrieve the effective diameter of the ice crystals and water droplets, the optical depth/ice water content and the ice fraction of the clouds. The comparison of the effective diameters and the ice water content with the measurement given by the ice camera and the radar represents one of the aims of the FIRCLOUDS (Far Infrared Radiative CLOUDS) project selected by the Italian PNRA (Programma Nazionale di Ricerche in Antartide), in order to validate the current cirrus clouds model. Furthermore the analysis of the whole database collected by REFIR-PAD will allow to obtain a statistics of the Antarctic clouds properties and studying the correlations between these clouds and the other atmospheric variables. [ABSTRACT FROM AUTHOR]