October 18, 2013

POLDER 2 - CLOUD PROPERTIES : The cloudy thermodynamical phase


The multidirectional polarization of POLDER:
a tool for deriving cloud thermodynamical phase

    The cloud thermodynamic phase is retrieved using multiangular measurements
    of the polarized reflectance (Riedi et al., GRL, 2000)
     Variations of cloud polarized reflectance with scattering angle
    exhibits very different features depending on particle shapes
    (liquid droplets vs ice crystals). The cloud phase algorithm uses
    4 major differences between these angular signatures:
    [1] a strong increase around 140 degrees corresponding to the cloud
    bow for liquid water clouds
    [2] a positive (resp. negative) slope of the polarized reflectance
    for liquid (resp. ice) clouds
    [3] a negative polarized reflectance below 100 degrees observed only
    for liquid clouds
    [4] a strong dispersion of polarized reflectance after 150 degrees
    due to supernumerary bows for liquid clouds

    The following figure presents the global map of retrieved cloud thermodymical phase for June 25, 2003. The Intertropical Convergence Zone is a region where ice phase is frequenty observed (Indonesia, West Africa), which is mainly due to cumulonimbus deep convection, injecting water vapor and ice particles into the upper atmosphere. On the contrary, stratocumulus clouds are very present off the coast of Angola or off South America and are generally detected as liquid water clouds from polarization measurements.

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