Angular variability of the liquid water cloud optical thickness retrieved from ADEOS-POLDER

The usual procedure for retrieving the optical thickness of liquid water cl ouds from satellite-measured radiances is based on the assumption of plane- parallel layers composed of liquid water droplets. This study investigates the validity of this assumption from Advanced Earth Orbiting Satellite-Pola rization and Directionality of the Earth's Reflectances (ADEOS-POLDER) obse rvations. To do that, the authors take advantage of the multidirectional vi ewing capability of the POLDER instrument, which functioned nominally aboar d ADEOS from November 1996 to June 1997. The usual plane-parallel cloud model composed of water droplets with an eff ective radius of 10 mum provides a reasonable approximation of the angular dependence in scattering at visible wavelengths from overcast liquid water clouds for moderate solar zenith angles. However, significant differences b etween model and observations appear in the rainbow direction and for the s mallest observable values of scattering angle (Theta <90<degrees>). A bette r overall agreement would be obtained for droplets with an effective radius of about 7-8 mum for continental liquid water clouds. On the other hand, c hanging the water droplet size distribution would not lead to a significant improvement for maritime situations. When horizontal variations in cloud o ptical thickness are considered by using the independent pixel approximatio n (IPA), a small improvement is obtained over the whole range of scattering angles but significant discrepancies remain for Theta <80<degrees>, that i s for large solar zenith angles in the forward-scattering direction. The re maining differences between various models based on the plane-parallel radi ative transfer and POLDER observations are thought to be due to variations in cloud shape.