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Abstract. Cloud phase recognition is important for cloud studies. Ice crystals correspond to physical process and properties that differ from those of liquid water drops. The angular polarization signature is a good mean to discriminate between spherical and nonspherical particles (liquid and ice phase, respectively). POLDER (Polarization and Directionality of Earth Reflectances) has been launched on the Japanese ADEOS platform in August 1996. Because of its multidirectional, multispectral, and multipolarization capabilities this new radiometer gives useful information on clouds and their influence on radiation in the shortwave range. The POLDER bidirectional observation capability provides the polarization signatures within a large range of scattering angles in three spectral bands centered on 0.443, 0.670, and 0.865 tzm with a spatial resolution of 6.2 km x 6.2 km. These original features allow to obtain some information both on cloud thermodynamic phase and on cloud microphysics (size/shape). According to POLDER airborne observations, liquid cloud droplets exhibit very specific polarization features of a rainbow for scattering angles near 140 ø. Conversely, theoretical studies of scattering by various crystalline particles and also airborne measurements show that the rainbow characteristics disappear as soon as the particles depart from the spherical shape. In the paper the POLDER algorithm for cloud phase classification is presented, as well as the physical principle of this algorithm. Results derived from the POLDER spaceborne version are also presented and compared with lidar ground-based observations and satellite cloud classification. This cloud phase classification method is shown to be reliable. The major limitation appears when thin cirrus clouds overlap the liquid cloud layer. In this case, if the cirrus optical thickness is smaller than 2, the liquid phase may be retrieved. Otherwise, the ice phase is correctly detected as long as cloud detection works. IntroductionClouds are a strong modulator of the shortwave and longwave component of the Earth's radiation budget [Ramanathan, 1987

[1] The new generation of spaceborne backscatter lidar systems, prefigured by the Lidar in-Space Technology Experiment (LITE) mission in September 1994, will give new insight on the vertical distribution of both aerosols and clouds in the atmosphere. This is especially of importance for aerosols over land, where retrievals from passive sensors are known to be more difficult because of the surface contribution. Here we analyze mineral dust aerosol transport events through a new approach coupling the active LITE and passive Meteosat-5 spaceborne observations. The Meteosat-derived aerosol optical thickness at 550 nm is shown to be a good boundary condition for the lidar inversion in order to retrieve both the aerosol backscatter to extinction ratio (BER) and the aerosol extinction vertical profile above the Tropical Atlantic Ocean (TAO) and the Mediterranean Sea. Sensitivity tests indicate that the aerosol scattering coefficient is retrieved within a 20% relative uncertainty. Air mass trajectories allow us to further retrieve the vertical profile of aerosol optical properties over the continent in the Saharan Heat Low (SHL) region using the BER determined over the ocean. Results confirm a large dispersion of the BER which is not attributed to errors in the method. This shows the need to account for such dispersion in the retrieval of dust aerosol optical thickness and aerosol impact on the earth radiative budget. The coupling between LITE and Meteosat-5 made here is shown to be interesting for an improvement of the direct dust aerosol forcing, and results should be improved by the CALIOP-MODIS synergy.

Abstract. Measurements of relative humidity (RH) and aerosol parameters (scattering cross section, size distributions and chemical composition), performed in ambient atmospheric conditions, have been used to study the influence of relative humidity on aerosol properties. The data were acquired in a suburban area south of Paris, between 18 and 24 July 2000, in the framework of the "Etude et Simulation de la Qualité de l'air en Ile-de-France" (ESQUIF) program. According to the origin of the air masses arriving over the Paris area, the aerosol hygroscopicity is more or less pronounced. The aerosol chemical composition data were used as input of a thermodynamic model to simulate the variation of the aerosol water mass content with ambient RH and to determine the main inorganic salt compounds. The coupling of observations and modelling reveals the presence of deliquescence processes with hysteresis phenomenon in the hygroscopic growth cycle. Based on the Hänel model, parameterisations of the scattering cross section, the modal radius of the accumulation mode of the size distribution and the aerosol water mass content, as a function of increasing RH, have been assessed. For the first time, a crosscheck of these parameterisations has been performed and shows that the hygroscopic behaviour of the accumulation mode can be coherently characterized by combined optical, size distribution and chemical measurements.