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  • Author or Editor: Gérard Brogniez x
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Guy Cautenet, Michel Legrand, Sylvie Cautenet, Bernard Bonnel, and Gérard Brogniez

Abstract

Simulations are carded out to verify a mesoscale model in order to perform sensitivity tests of satellite response to atmospheric dust content. The model chosen is the mesoscale model of Colorado State University with a modified radiation parameterization in order to take atmospheric dust content into account. Downward and upward longwave irradiances are estimated using a 25-interval model. The shortwave pan of the spectrum is processed by a very fast, highly parameterized, single-interval code. Tests using experimental data gathered during the Etude de la Couche Limite Atmosphérique Tropicale Sèche (ECLATS) experiment performed during the 1980 dry season near Niamey (Niger, West Africa) prove that dust content is satisfactorily handled. Three 24-h simulations performed under various meteorological and turbidity conditions show that ground surface energy exchanges are satisfactorily described, so that surface temperature is predicted with a standard deviation of about 1°C. Vertical profiles of computed air temperature and shortwave and longwave irradiances are also realistic.

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Hélène Chepfer, Philippe Goloub, James Spinhirne, Pierre H. Flamant, Mario Lavorato, Laurent Sauvage, Gérard Brogniez, and Jacques Pelon

Abstract

Bidirectional polarized reflectances measured with the POLDER-1 instrument on board Advanced Earth Observing Satellite-1 have been used to infer cloud altitude and thermodynamical phase (ice/liquid) at a global scale. This paper presents a validation of these properties for cirrus clouds. The validation presented here is based on comparisons between POLDER-1 retrievals and measurements collected with a ground-based lidar network. The scale differences between POLDER measurements and lidar data are treated by selecting homogeneous and stable cloud layers.

These comparisons show that the cloud altitude retrieval with POLDER is valid for optically thick cloud, and nonvalid for semitransparent and thin cirrus clouds. The limitations of the cloud altitude retrieval method are analyzed by using both comparisons between POLDER and lidar and simulations of the bidirectional polarized reflectances performed with a radiative transfer code to assess a threshold of validity of the POLDER retrieval method. The comparisons of lidar and POLDER data show that the cloud thermodynamical phase (ice/liquid) retrieval is satisfactory, and examples of cloud thermodynamical phase retrieval are presented as a function of cloud temperatures.

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Artemio Plana-Fattori, Gérard Brogniez, Patrick Chervet, Martial Haeffelin, Olga Lado-Bordowsky, Yohann Morille, Frédéric Parol, Jacques Pelon, Antoine Roblin, Geneviève Sèze, and Claudia Stubenrauch

Abstract

The characterization of high clouds as performed from selected spaceborne observations is assessed in this article by employing a number of worldwide ground-based lidar multiyear datasets as reference. Among the latter, the ground lidar observations conducted at Lannion, Bretagne (48.7°N, 3.5°W), and Palaiseau, near Paris [the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA) observatory: 48.7°N, 2.2°E], both in France, are discussed in detail. High-cloud altitude statistics at these two sites were found to be similar. Optical thicknesses disagree, and possible reasons were analyzed. Despite the variety of instruments, observation strategies, and methods of analysis employed by different lidar groups, high-cloud optical thicknesses from the Geoscience Laser Altimeter System (GLAS) on board the Ice, Cloud and land Elevation Satellite (ICESat) were found to be consistent on the latitude band 40°–60°N. Respective high-cloud altitudes agree within 1 km with respect to those from ground lidars at Lannion and Palaiseau; such a finding remains to be verified under other synoptic regimes. Mean altitudes of high clouds from Lannion and Palaiseau ground lidars were compared with altitudes of thin cirrus from the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) Path-B 8-yr climatology for a common range of optical thicknesses (0.1–1.4). Over both sites, the annual altitude distribution of thin high clouds from TOVS Path-B is asymmetric, with a peak around 8–9.5 km, whereas the distribution of high clouds retrieved from ground lidars seems symmetric with a peak around 9.5–11.5 km. Additional efforts in standardizing ground lidar observation and processing methods, and in merging high-cloud statistics from complementary measuring platforms, are recommended.

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Albert Ansmann, Jens Bösenberg, Gérard Brogniez, Salem Elouragini, Pierre H. Flamant, Karlheinz Klapheck, Holger Linn, Louis Menenger, Walfried Michaelis, Maren Riebesell, Christoph Senff, Pierre-Yves Thro, Ulla Wandinger, and Claus Weitkamp

Abstract

Four lidars located roughly 75 km from each other in the inner German Bight of the North Sea, were used to measure geometrical and optical properties of cirrus clouds during the International Cirrus Experiment 1989 (ICE '89). A complete cirrus life cycle was observed simultaneously with three lidan during a case study on 18 October 1989. Time series of particle backscatter, depolarization-ratio height profiles, cloud depth, optical thickness, and of the cirrus extinction-to-backscatter, or lidar, ratio describe the evolution of the cloud system. A two-wavelength lidar measurement was performed and indicates wavelength independence of ice-crystal scattering. The optical and geometrical depths of the cirrus were well correlated and varied between 0.01 and 0.5 and 100 m and 4.5 km, respectively. Although the evolution of the cloud deck was similar over the different observation sites, cirrus geometrical, scattering, and microphysical properties were found to vary considerably within the lidar network. A statistical analysis of ice-cloud properties is performed based on 38 different cirrus cases sampled during ICE '89. Cirrus formation was found to start at the tropopause in most cases. Ice clouds, measured at high midlatitudes (around 54°N), were thin with mean optical and geometrical depths mainly below 0.4 and 2 km, respectively. A good correlation between mean cloud optical and geometrical thickness, and a weak decrease of the mean optical depths with temperature was observed.

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