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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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Gunnar Myhre, Frode Stordal, Mona Johnsrud, Alexander Ignatov, Michael I. Mishchenko, Igor V. Geogdzhayev, Didier Tanré, Jean-Luc Deuzé, Philippe Goloub, Teruyuki Nakajima, Akiko Higurashi, Omar Torres, and Brent Holben

Abstract

For an 8-month period aerosol optical depth (AOD) is compared, derived over global oceans with five different retrieval algorithms applied to four satellite instruments flown on board three satellite platforms. The Advanced Very High Resolution Radiometer (AVHRR) was flown on board NOAA-14, the Ocean Color and Temperature Scanner (OCTS) and the Polarization and Directionality of the Earth's Reflectances (POLDER) on board the Advanced Earth Observing Satellite (ADEOS), and the Total Ozone Mapping Spectrometer (TOMS) on board the Earth Probe satellites. The aerosol data are presented on the same format and converted to the same wavelength in the comparison and can therefore be a useful tool in validation of global aerosol models, in particular models that can be driven with meteorological data for the November 1996 to June 1997 period studied here. Large uncertainties in the global mean AOD are found. There is at least a factor of 2 difference between the AOD from the retrievals. The largest uncertainties are found in the Southern Hemisphere, and the smallest differences mostly near the continents in the Northern Hemisphere. The largest relative differences are probably caused by differences in cloud screening.

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