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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]

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]