POLDER 2 - CLOUD PROPERTIES: Cloud Phase

Cloud phase determination is based on drastic differences between angular polarization signatures produced by spherical and non spherical particles, in other words by spherical liquid droplets and non spherical ice crystals (Goloub et al., 2000). Our method is entirely based on the analysis of angular behavior of polarized radiance in the near infrared (865nm). As such, it takes advantage of two of POLDER most original features, namely its multi-angle and polarization observations capabilities.
From extended analysis and validation of ADEOS 1 - POLDER cloud phase product a new and improved algorithm has been designed to take better advantage of POLDER original multi-angular and polarization capabilities. Angular polarization signatures of clouds at 865 nm are being analyzed directly at a 3 x 3 pixels resolution which provides for more robust determination in case of broken or inhomogeneous cloud fields. Also, the angular signature is now analyzed more thoroughly. More informations are derived from the angular signature and those are now recombined using a logic array decision approach that enables to make the final phase determination and to attribute a quality index (confidence and/or flag for particular cases) to the retrieval. This latest improvement makes the phase product more meaningful than the usual and previously derived liquid/ice index.

In addition to this, a microphysical index is derived for ice clouds only, based on the polarized radiance at 865 nm. As demonstrated by various studies (Chepfer et al. 2001, C-Labonnote et al., 2001, Riedi et al., 2001) polarization radiance is very sensitive to ice crystal shape. This experimental product is expected to provide information about ice crystal shape/size habits at global scale but will require extended investigation in order to interpret and assess its physical meaning