The two-stream delta-Eddington approximation to simulate the far infrared Earth spectrum for the simultaneous atmospheric and cloud retrieval
Authors: Di Natale G.; Palchetti L.; Bianchini G.; Ridolfi M.
Autors Affiliation: INO CNR, Via Madonna Piano 10, Florence, Italy; Univ Bologna, Dipartimento Fis & Astron, Via Irnerio 46, Bologna, Italy
Abstract: 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 delta-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 pm and optical depths between 0.1 and 10, our model shows biases smaller than 0.4 mW/(m(2) sr cm(-1)) in upwelling radiance simulations, and biases smaller than 0.3 mW/(m(2) 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. (C) 2020 Elsevier Ltd. All rights reserved.
Journal/Review: JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER
Volume: 246 Pages from: 106927-1 to: 106927-11
More Information: This research was supported by the Italian PNRA (Programma Nazionale di Ricerche in Antartide) and the Institut polaire francais Paul Emile Victor (IPEV). More specifically the research was conducted as a part of the sub-project FIRCLOUDS (Far Infrared Radiative Closure Experiment For Antarctic Clouds) and DoCTOR (DOme C Tropospheric ObserveR). Data and information on radio sounding measurements were obtained from the IPEV/PNRA Project “Routine Meteorological Observation at Station Concordia”. We thank the mentioned institutions for supplying information about other measurements available at Concordia station. We also thank Dr. Massimo Del Guasta for providing the cloud lidar profile at DomeC and two anonymous reviewers for improving our work.KeyWords: Two-streams delta-Eddington approximation; Cloud retrieval; Cirrus clouds; Far-infrared spectral radianceDOI: 10.1016/j.jqsrt.2020.106927