HR: 0800h
AN: A21B-0431    [Abstracts]
TI: On the use of Polarization and Backscatter Features to Retrieve Liquid Water Clouds Microphysical Properties
AU: * C.-Labonnote, L
EM: labon@loa.univ-lille1.fr
AF: LOA, U.S.T.L. Bat. P5, Villeneuve d'Ascq, 59655, France
AU: Riedi, J
EM: riedi@loa.univ-lille1.fr
AF: LOA, U.S.T.L. Bat. P5, Villeneuve d'Ascq, 59655, France
AB: Multiangle polarized measurements can be used to observe cloud bow together with its supernumerary bows over liquid clouds. As demonstrated by Breon and Goloub [1998], such measurements around cloud bow, are suitable to retrieve effective radius and effective variance of liquid droplets distribution. Recently the method has been applied to POLDER (Polarization and Directionality of the Earth Reflectances) data, and retrieved parameters were compared to MODIS (Moderate Resolution Imaging Spectroradiometer) retrievals using a bispectral technique [Nakajima and King, 1990], [Platnick et al, 2003]. Results of this study [Breon and Doutriaux, 2005] showed an apparent systematic discrepancy between the two techniques and conclusions were directed toward erroneous assumptions in the microphysical models used in MODIS retrievals. The A-Train mission has been providing simultaneous observations between POLDER3/Parasol and MODIS/Aqua for more than two years and it is of primary importance to understand correctly the information provided by each instrument if we aim at using both instrument in a synergistic way. We are revisiting here the use of multidirectionnal observations of cloud polarized reflectances in the backscatter direction for liquid clouds microphysical properties retrieval. Based on a variational method and a simple forward model, we present evidence that observed differences between MODIS and POLDER retrievals can be explained by taking into account cloud field heterogeneities. We show in particular that techniques using single scattering features are subject to potential biases that might not appear clearly if the assumption of homogeneous cloud is used. Based on single scattering approximation, full computation of reflectance field, and data from the MODIS and POLDER instrument we provide here an explanation to reconcile these apparently incompatible retrievals.
DE: 0319 Cloud optics
DE: 0321 Cloud/radiation interaction
DE: 0360 Radiation: transmission and scattering
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting