11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002
Development of light scattering algorithms for non-spherical cloud particles: Approximation and exact solutions
Takashi Y. Nakajima, NASDA, Tokyo, Japan; and T. Nakajima, Y. Mano, K. Yoshimori, and A. A. Kokhanovsky
Poster PDF (667.1 kB)
Two light scattering algorithms that calculate scattering properties of arbitrary shape non-spherical ice cloud particles were developed. One algorithm is approximation method based on GOA approach that is reliable among relatively large size parameter, the other algorithm is an exact method based on a Maxwell-originated integral equations approach SIEM/M that is applicable to small and moderate size parameters.

The phase functions of homogeneous ice hexagonal columns calculated by GOA have been integrated with a log-normal particle size distribution and used as an input to the radiation transfer program to calculate expected measured radiances of clouds from spaceborne multispectral imagers. It was found that a difference of calculated reflected radiances of hexagonal column particle and spherical particles is large. This result indicates that one cannot apply the exact spherical (Mie) theory, which is valid for retrievals only in the case of spherical particles, to calculate light scattering properties of non-spherical ice clouds with both visible and near-infrared wavelength. Near-global analyses of ice clouds were performed by using daytime NOAA AVHRR data and retrieval algorithms of a solar reflection method with light scattering characteristics of clouds consist of homogeneous, hexagonal column ice particles calculated by a GOA. The retrieval results show that the distribution of optical thickness is different between in area of, lower latitude, higher latitude summer hemisphere, and higher latitude winter hemisphere. The globally averaged optical thickness of the thick ice clouds with optical thickness less than 64 was about 14. Effective radius could not be retrieved in this analysis due to the fatal differences in near-infrared radiances obtained by AVHRR observations and radiative transfer calculations. These differences in radiances led to the result that at this wavelength, most ice particles were relatively in small size parameters and not in the geometrical optics domain.

An exact method based on SIEM/M was developed to overcome the problems revealed in GOA approach. A good agreement was obtained by comparing SIEM/M results and exact spherical (Mie) results for spherical particle. Phase function of randomly oriented hexagonal column in size parameter alfa=20 shows a remarkable halo phenomenon that uniquely appears in hexagonal structure of the particles. We plan to apply the SIEM/M (to moderate alfa and GOA (to large alfa ) results of hexagonal columns to the ice cloud remote sensing by making use of visible to infrared imager such as AVHRR, GLI, and MODIS.

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