FEASIBILITY STUDY OF DERIVATION OF CIRRUS INFORMATION USING POLARIMETRIC MEASUREMENTS FROM SATELLITE

Monitoring techniques for deriving cirrus information over the ocean using polarimetric measurements from satellite are studied. Radiance and degree of linear polarization of the upwelling radiation at the top of a model atmosphere-ocean system are computed at the wavelength of 0.88-mu-m with the aid of the doubling-adding method, and where cirrus cloud is assumed to be composed of water droplets or randomly oriented hexagonal ice crystals (columns and plates). The computational results show, in particular, the following features. Degree of polarization observed at the Brewster angle in the solar plane is suitable to derive cloud optical thickness (tau) for large solar zenith angles, particularly in the case of tau < approximately 5 while the radiance off the sunglint is suitable to infer tau in the range of tau < approximately 30. However, for improvement of accuracy, cloud information such as thermodynamic phase, size distribution, and shape of cloud particles should be separately obtained, in particular for utilization of degree of polarization. The difference of degree of polarization appears between water particles and hexagonal ice crystals at the top of the atmosphere when the scattering angle is within 50-100-degrees. Using this property, it should be possible to distinguish the thermodynamic phase of cloud particles. Furthermore, the feasibility of discrimination of the shape of the hexagonal ice crystals using the position of the neutral point is discussed, where phase angle is shown to shift to the backscattering direction at tau < 10. Hence, it is necessary to obtain tau separately.