Simultaneous retrieval of water vapour, temperature and cirrus clouds properties from measurements of far infrared spectral radiance over the Antarctic Plateau

Year: 2017

Authors: Di Natale G., Palchetti L., Bianchini G., Del Guasta M.

Autors Affiliation: Ist Nazl Ottica CNR, Sesto Fiorentino, Italy.

Abstract: 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 delta-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 con-strain 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 dŽUrville and in the Arctic region.

Journal/Review: ATMOSPHERIC MEASUREMENT TECHNIQUES

Volume: 10 (3)      Pages from: 825  to: 837

More Information: The deployment of REFIR-PAD in Antarctica was supported by the Italian National Program for Research in Antarctica PNRA (Programma Nazionale di Ricerche in Antartide) under the following projects: 2009/A04.03, 2013/AC3.01 and 2013/AC3.06. The authors are grateful to the research group of the Institute of Applied Physics Nello Carrara (CNR-Florence) composed by Bruno Carli, Simone Ceccherini, Marco Gai, Samuele Del Bianco, Ugo Cortesi, Marco Ridolfi, Piera Raspollini, Flavio Barbara, and Luca Sgheri of the Institute for the Applications of Calculus (CNR-Florence). for the precious and fruitful discussions.
KeyWords: Air temperature; Cirrus; Estimation method; Infrared radiation; Measurement method; Optical depth; Radionuclide; Vertical profile; Water vapor, Antarctica; Concordia Station; Dumont d’Urville; East Antarctica
DOI: 10.5194/amt-10-825-2017

Citations: 14
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