Evaluation of Yellow Sand Dust in the Vegetation Index Estimated from Satellite-level Data

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The yellow sand dust (called "Kosa" in Japan) transported from the deserts in the northern part of China is regarded as the main source of atmospheric aerosols for East Asia in the late winter and spring. The dust particles in the atmosphere have significant effects on retrieving the surface information such as the vegetation index from satellite-level data, because the radiance received by the satellite sensor includes the skylight scattered diffusely by the yellow sand dust.(1) The distribution of hazy Kosa clouds and the optical properties of yellow sand dust were estimated, using ADEOS/POLDER data taken on April 12 and 13, 1997.The transport paths and the global mass distribution of yellow sand dust were estimated by means of the long-range transport simulation of dust particles. The result of the long-range transport simulation shows that the regions of the high concentration of yellow sand over the Sea of Japan obtained from the simulation almost correspond to the hazy Kosa regio … More ns appeared in the POLDER image. At several points selected from the hazy regions in the POLDER image, the observed reflectance's were compared with those obtained from the multiple scattering processes of light in the atmosphere-ocean system. As a result, it was found that at the 565nm channel, the refractive index of yellow sand dust is in the range from 1.45 to 1.6, and the optical thickness of atmosphere τ(565nm) is 0.42 to 0.56.(2) Topographic effects included in one pixel of NOAA/AVHRR data were evaluated.In the remote sensing of mountainous terrain, the radiance received by the AVHRR sensor is affected by the occlusion by adjacent terrain and the shadow cast by adjacent terrain. In this study, the radiance variation caused by the occlusion and shadow will be called a topographic effect. The Monte Carlo method was used to evaluate topographic effects. The 50m-mesh digital terrain model was used as the ground surface. The ratio, R, of the radiance received in rugged terrain to that in the flat terrain was computed. The following results were obtained. At the visible channel (Ch1) of AVHRR data, the value of R is larger than 0.9, not depending on the roughness of the surface. At the near infrared channel (Ch2), the value of R changes from 0.85 to 1.0, depending on the roughness of the surface when the viewing angles is smaller than 40 degrees. This indicates that the topographic effect included in AVHRR data is not so much significant. Less