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

来自中国北部沙漠的黄沙（日本称“沙”）被认为是东亚地区冬末和春季大气气溶胶的主要来源。由于卫星传感器接收到的辐射包含了黄沙尘漫射散射的天光，因此大气中的沙尘粒子对从星级数据中提取植被指数等地表信息有重要影响。(1)利用1997年4月12日和13日的ADEOS/POLDER资料，估算了雾霾科沙云的分布和黄沙尘的光学性质。通过沙尘颗粒的远程输运模拟，估算了黄沙尘的输运路径和全球质量分布。长程输运模拟结果表明，模拟得到的日本海上空黄沙浓度较高的区域与雾蒙蒙的科沙地区基本一致。在POLDER图像的朦胧区域中选择几个点，将观测到的反射率与大气-海洋系统多次散射过程得到的反射率进行了比较。结果发现，在565nm通道处，黄沙尘的折射率在1.45 ~ 1.6之间，大气τ(565nm)的光学厚度为0.42 ~ 0.56。(2)对NOAA/AVHRR数据每像元包含的地形效应进行评价。在山地地形遥感中，AVHRR传感器接收到的辐射受到相邻地形遮挡和相邻地形投下阴影的影响。在本研究中，遮挡和阴影引起的亮度变化将被称为地形效应。采用蒙特卡罗方法对地形效应进行了评价。地面采用50m网格数字地形模型。计算了崎岖地形和平坦地形的辐射比R。得到了以下结果：在AVHRR数据的可见通道（Ch1）， R值大于0.9，与表面粗糙度无关。在近红外通道（Ch2），当视角小于40°时，R值随表面粗糙度的不同而变化，取值范围为0.85 ~ 1.0。这说明AVHRR数据中包含的地形效应并不十分显著。少

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